FAQ & Troubleshooting

A guide to common issues and solutions you may encounter during integration.

Frequently Asked Questions (FAQ)

Q. What if the user has no gas (ETH/MATIC) at all for the first Approve?

For standard ERC-20 tokens, approve() necessarily incurs a transaction gas fee. Solutions and recommendations:

1. Use a Permit-supported token (strongly recommended): By using ERC-20 tokens that natively support EIP-2612 Permit (such as USDC), even the initial Approve can be replaced with a Permit signature, enabling 100% signature-based gasless operation.
2. Operate a merchant faucet: You can airdrop a small amount of MATIC to new users via a separate merchant service, though this involves development and operational costs.

Q. I did not receive the payment completion event via Webhook.

This may be caused by a temporary network issue. The system relayer performs retries at set intervals when Webhook delivery fails (i.e., no 200 OK received). As a safety net, we recommend implementing a 'Refresh Payment Status' button on the merchant client (or server) that polls the GET /payments/:id API.

Troubleshooting

1. Signature Verification Failed (INVALID_SIGNATURE Error)

This occurs when INVALID_SIGNATURE is returned when submitting a signature to the Relay API. Checkpoints:

• Verify that the chainId in the EIP-712 domain parameters matches the chain ID of the network the user's MetaMask is currently connected to.
• Check that the forwarderAddress value used matches the value received in the payment session API response.
• If any value in the signed data structure — such as the payment amount or paymentId — is changed, the signature becomes invalid.

2. Revert After Relay Submission (Payment Status FAILED)

This occurs when the relayer submits a transaction to the mempool but the smart contract reverts. Checkpoints:

• Insufficient Approve: Occurs when a gasless signature is submitted without sufficient allowance for a token that does not support Permit.
• Insufficient Balance: The token balance in the wallet is less than the payment amount.
• Nonce Issue: If a user generates and submits multiple signatures in very rapid succession and a previous signature's nonce is resubmitted, the relay will reject it.

3. Client Error Messages

Situation A: "Unrecognized chain ID" or "Unsupported network"

The user is attempting to sign while their wallet is connected to a different network (e.g., Ethereum mainnet) instead of Polygon Amoy.

• Solution: Use wagmi's useSwitchChain() to switch the network to the contract's target chain before generating the signature.

Situation B: "CORS error"

This appears when the client browser calls the POST /payments endpoint using a Public Key (pk_xxx).

• Solution: The Origin header domain is not correctly registered in the admin whitelist. Request the SoloPay operations team to add the domain as an allowed origin.

Other API Error Codes

The many status codes returned during payment session creation and relay requests each have a clear cause. For more details, refer to the API Error Code Reference.