there's not much else we can do from here. you can try asking the mods on the coinbase subreddit: https://www.reddit.com/r/CoinBase

or you could try asking at https://ethereum.stackexchange.com/ as well. maybe there's something we're not thinking of.

the way i see it, there's a few possibilities:

1a. the merchant's seed is wrong
1b. you sent tokens to an address not in the merchant's wallet
2a. the reddit info is incorrect = the derivation path is wrong
2b. you are inputting the derivation path incorrectly
3. you haven't derived enough addresses or scanned for the address correctly