forwarder.sol

contract Forwarder {
  address public destination;

  constructor(address _destination) public {
    destination = _destination;
  }

  function flushERC20(address tokenContractAddress) public {
    IERC20 tokenContract = ERC20(tokenContractAddress);
    uint256 forwarderBalance = tokenContract.balanceOf(address(this));
    tokenContract.transfer(destination, forwarderBalance);
  }
}

Shouldn't it be IERC20 tokenContract = ERC20(tokenContractAddress);? Isn't it a typo?

Yes, you are correct! Updated the gist, thank you for the note!

How does the forwarder contract know which token to forward?
Or is that determined via a separate call to flushERC20(address tokenContractAddress)?
It looks like Coinbase Commerce provides the same address, e.g. 0x3d0a...2c7a, for all three payment methods ETH, DAI, USDC.
Yet, TX event logs for a successful transactions don't seem to call flushERC20 at all.
How does this work?

This part is a bit complicated.

All interactions Coinbase has are through the factory contract. The first time you pay to an address, that factory will create a Forwarder & flush the ERC20 token indicated in the argument. The second time you pay to the same address, the factory will call a different method that will just flush the tokens (again, the token is indicated in the argument).

This is also made a bit more difficult, since you are actually calling the original Forwarder and not the proxy copies; so the tx logs might look funky at a first glance.

ETH is handled differently from the ERC20 tokens.

Hi
I tried to pay the customer via coinbase commerce with this forwarder contract but I accidentally sent it via BEP20 instead of ERC20, what needs to be done now in order to flush the ETH PEG tokens to the merchant final address?