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Gas Fees Explained

reply.cash uses the Kora relayer to enable gasless transactions, allowing you to pay SOL gas fees using USDC from your wallet balance. You no longer need SOL in your wallet to use reply.cash.

How Gas Fees Work with Kora Relayer​

No SOL Required​

You only need USDC:

  • ✅ Top up only USDC - No need to acquire or manage SOL
  • ✅ Automatic gas payment - Gas fees are paid from your USDC balance
  • ✅ Privacy preserved - Use privacy services like PrivacyCash without compromising anonymity
  • ✅ Seamless experience - Just ensure you have sufficient USDC balance

How the Relayer Works​

reply.cash uses Kora, Solana's signing infrastructure, to handle gas fees:

  1. You initiate a transaction - Send USDC to mobile money or banks
  2. Relayer calculates gas cost - Determines the SOL transaction fee needed
  3. 10% margin applied - Relayer charges a 10% margin on the SOL transaction fee
  4. USDC deducted - Equivalent amount in USDC (including margin) is deducted from your balance
  5. Transaction processed - Relayer pays SOL gas fees and processes your transaction

Example:

  • SOL gas fee: 0.000005 SOL ($0.001 USD)
  • 10% margin: ~$0.0001 USD
  • Total USDC deducted: ~$0.0011 USD from your USDC balance
  • No SOL needed in your wallet

Gas Fee Pricing​

Current Pricing Structure​

Relayer Fee:

  • Base cost: Actual SOL transaction fee (typically < $0.01)
  • Margin: 10% of the SOL transaction fee
  • Total cost: Base SOL fee + 10% margin

Example Costs:

Transaction TypeSOL Gas Fee10% MarginTotal USDC Cost
Simple USDC transfer~$0.001~$0.0001~$0.0011
Cross-chain transfer~$0.002~$0.0002~$0.0022
Local payment rails~$0.001~$0.0001~$0.0011

💡 Tip: Gas fees are extremely low on Solana. Even with the 10% margin, total costs remain under $0.01 per transaction.

Factors Affecting Gas Costs​

  • Network congestion (usually minimal on Solana)
  • Transaction complexity (more operations = slightly higher fees)
  • Current SOL price (affects USD equivalent)

Privacy Benefits​

Why This Matters for Privacy​

Traditional Problem:

  • To use Solana, you typically need SOL in your wallet
  • Getting SOL often requires sending it from another address
  • This can link your addresses and compromise privacy

With Kora Relayer:

  • ✅ Top up only USDC - Use privacy services like PrivacyCash
  • ✅ No SOL transfers needed - Never need to send SOL from another address
  • ✅ Privacy preserved - Your transaction history remains private
  • ✅ Anonymity maintained - No address linking through SOL transfers

Use Cases:

  • Top up with privacy-focused services (PrivacyCash, etc.)
  • Maintain transaction privacy
  • Avoid address linking
  • Use reply.cash without compromising anonymity

Supported Tokens​

Current Support​

Currently Supported:

  • ✅ USDC - Primary token for gas payment

Future Support​

Coming Soon:

  • 🔜 USDT - Pay gas with USDT if you have USDT in your wallet
  • 🔜 Any token - Relayer will automatically use whatever token you have available
  • 🔜 Multi-token support - Use the best available token for gas payment

📚 Learn more: Kora Relayer Documentation - Solana's signing infrastructure

How It Works​

Step-by-Step Process​

  1. You have USDC balance

    • Top up your wallet with USDC only
    • No SOL required

  2. Initiate transaction

    • Send USDC to mobile money or banks
    • Or perform any reply.cash transaction

  3. Relayer handles gas

    • Kora relayer calculates required SOL gas fee
    • Applies 10% margin
    • Deducts equivalent USDC from your balance

  4. Transaction completes

    • Your USDC is sent to recipient
    • Gas fees paid automatically
    • No manual intervention needed

Who Pays Gas?​

Current System:

  • Senders pay gas fees (deducted from USDC balance via relayer)
  • Recipients do not pay gas fees when receiving payments through local rails
  • For cross-chain transfers, gas is paid by the sender

No SOL Required:

  • You never need SOL in your wallet
  • All gas fees are paid from your USDC balance
  • Relayer handles all SOL transactions

Cost Comparison​

Traditional Method (Without Relayer)​

What you needed:

  • USDC for the transfer
  • SOL for gas fees (separate acquisition)
  • Multiple steps to get SOL
  • Potential privacy compromise

Costs:

  • SOL gas fee: ~$0.001
  • Time/effort to acquire SOL
  • Privacy risk from SOL transfers

With Kora Relayer​

What you need:

  • USDC only (for transfer + gas)

Costs:

  • SOL gas fee: ~$0.001
  • 10% margin: ~$0.0001
  • Total: ~$0.0011 USD
  • No SOL acquisition needed
  • Privacy preserved

💡 Benefit: The small 10% margin is worth it for the convenience and privacy benefits.

Technical Details​

About Kora​

Kora is Solana's signing infrastructure that enables:

  • Gasless transactions - Pay fees in any token (USDC, USDT, etc.)
  • Trusted signing - Secure transaction processing
  • Production ready - Audited and secure
  • Flexible deployment - Used by reply.cash for reliable gas handling

Key Features:

  • ✅ Secure validation and rate limiting
  • ✅ Full Token-2022 support
  • ✅ Redis caching for performance
  • ✅ HMAC and API key authentication
  • ✅ Prometheus metrics and monitoring

Source: Kora GitHub Repository

Relayer Architecture​

  • Language: Rust (Kora) with TypeScript SDK
  • Protocol: JSON-RPC 2.0
  • Signers: Secure key management
  • Authentication: Secure API authentication
  • Deployment: Production-ready infrastructure

Troubleshooting​

"Insufficient funds for transaction" Error​

Solutions:

  • ✅ Check that you have sufficient USDC balance (amount + gas fees)
  • ✅ Ensure your USDC balance covers both the transfer amount and gas fees
  • ✅ Gas fees are typically < $0.01, so ensure you have a small buffer
  • ✅ Example: For a $100 transfer, ensure you have at least $100.01 USDC

Transaction Stuck or Pending​

Solutions:

  • ✅ Verify you have sufficient USDC balance
  • ✅ Check Solana network status
  • ✅ The relayer handles all SOL transactions automatically
  • ✅ Contact support if transaction remains pending

Understanding Gas Costs​

Why the 10% margin?

  • Covers relayer infrastructure costs
  • Ensures reliable transaction processing
  • Maintains service availability
  • Still extremely low cost (< $0.01 per transaction)

Is it worth it?

  • ✅ Yes - Convenience of not needing SOL
  • ✅ Yes - Privacy benefits
  • ✅ Yes - Seamless user experience
  • ✅ Yes - Still very low cost

Gas for Different Transaction Types​

USDC Transfers​

  • Gas fee: ~$0.001 USD (paid in USDC via relayer)
  • 10% margin: ~$0.0001 USD
  • Total cost: ~$0.0011 USD
  • No SOL needed - All paid from USDC balance

Cross-Chain Transfers​

  • Gas fee: Slightly higher (~$0.002 USD) due to additional operations
  • 10% margin: ~$0.0002 USD
  • Total cost: ~$0.0022 USD
  • Paid from USDC balance automatically

Local Payment Rails​

  • Blockchain gas: ~$0.001 USD (paid in USDC via relayer)
  • 10% margin: ~$0.0001 USD
  • Local fees: None for recipients
  • Total cost: ~$0.0011 USD from your USDC balance

Private Transfers​

  • Gas costs: Similar to standard transfers
  • Privacy features: Don't significantly increase gas requirements
  • All paid from USDC: Via relayer system

Best Practices​

Managing Your USDC Balance​

  • ✅ Keep a small buffer - Add ~$0.10-0.50 extra USDC for gas fees
  • ✅ Monitor balance - Ensure sufficient USDC for transfers + gas
  • ✅ No SOL needed - Focus only on maintaining USDC balance
  • ✅ Privacy first - Use privacy services for top-ups without worry

Privacy Tips​

  • ✅ Top up with privacy services - Use PrivacyCash or similar services
  • ✅ No SOL transfers - Never need to send SOL from another address
  • ✅ Maintain anonymity - Your transaction history stays private
  • ✅ Single wallet - Use one wallet with USDC only

Frequently Asked Questions​

Do I still need SOL in my wallet?​

No! You only need USDC. The Kora relayer handles all SOL gas fees automatically, deducting the cost (with 10% margin) from your USDC balance.

How much does the relayer cost?​

The relayer charges a 10% margin on the actual SOL transaction fee. Since Solana gas fees are extremely low (< $0.01), the total cost remains under $0.01 per transaction.

Can I pay gas with other tokens?​

Currently, gas is paid with USDC. In the future, the relayer will support paying with USDT or any token you have available in your wallet.

Why use a relayer instead of SOL?​

Benefits:

  • ✅ Privacy - No need to send SOL from another address
  • ✅ Convenience - Only need USDC, not SOL
  • ✅ Simplicity - One token to manage
  • ✅ Anonymity - Maintain privacy with services like PrivacyCash

Is the 10% margin expensive?​

No! The margin is extremely small:

  • SOL gas fee: ~$0.001
  • 10% margin: ~$0.0001
  • Total: ~$0.0011 (less than 1 cent)

The convenience and privacy benefits far outweigh this minimal cost.

How does this help with privacy?​

Traditional flow:

  1. Get USDC from privacy service
  2. Need SOL for gas
  3. Send SOL from another address → Privacy compromised

With relayer:

  1. Get USDC from privacy service
  2. Use reply.cash directly → Privacy preserved

What if I want to use SOL directly?​

You can still use SOL directly if you prefer, but the relayer provides a better experience for most users, especially those concerned about privacy.

See Also​

Pay SOL gas fees with USDC via Kora relayer. No SOL needed - top up only USDC and use reply.cash while maintaining your privacy.