Swiftrail

SwiftRail is a gasless USDC checkout stack for Arbitrum Sepolia.

Customers sign once with USDC permit and do not pay ETH gas. A relayer sponsors gas, takes a small fee, and settles net USDC to the merchant.

How everything fits together

Think of this as 5 parts working together:

1. frontend: the shop UI where user clicks buy.

2. sdk: helps connect wallet and enforce Arbitrum Sepolia network.

3. relayer: backend service that sends on-chain transactions and pays gas.

4. payment contract: verifies permit and pulls USDC from customer.

5. USDC token: where permit nonce/balance and token transfers happen.

Payment sequence:

1. User clicks buy in frontend.

2. Frontend asks relayer for fee quote (POST /quote).

3. Frontend calls sdk.checkout(...).

4. SDK prompts user to sign USDC permit typed-data in wallet.

5. SDK submits permit payload to relayer (POST /submit-payment).

6. Relayer calls contract processPayment(...) (tx #1).

7. Contract does permit + transferFrom(user -> fee_collector).

8. Relayer waits for tx #1 confirmation.

9. Relayer transfers net USDC to merchant (tx #2).

10. Relayer waits for tx #2 confirmation.

11. Frontend refreshes dashboard (GET /dashboard/:merchant).

This is exactly why the logs show:

1. Payment transaction submitted

2. Payment transaction confirmed

3. Merchant settlement tx submitted

4. Merchant settlement tx confirmed