It's important to understand how Coinbase handles cryptocurrency transactions to ensure any returns reach the sender's account. Coinbase does not support cryptocurrency being returned directly to an address it was sent "from" (in the cryptocurrency world, we call these input addresses).
When a Coinbase user sends cryptocurrency from their crypto balance, the address it comes "from" is one of Coinbase's many hot wallet addresses. Any crypto sent back to that address would be sent to Coinbase, not your own crypto balance.
For returns being sent back to a Coinbase customer, you can:
Send the return back to the sender's registered Coinbase email address and our system will match this to their account
Ask the sender to sign in, go to a crypto currency balance of their choice, and generate a new address buy selecting Receive
Ask the sender to provide an address from their Coinbase account; each address here is capable of receiving and matching a refund
Once you have a cryptocurrency or email address provided for refund, you can send the funds.
Please note, Coinbase only supports receiving specific cryptocurrencies. Any cryptocurrency that is sent to the address of another cryptocurrency will be lost (if you send BTC to a BCH address, these funds will not be recoverable).
If the party you are trying to return funds to is not using Coinbase, you will need to reach out to them directly in order to get an address that can accept returns.
More information on Coinbase transactions is included in the following article:Why can't I see my transaction in the blockchain?