It doesn't sound like Customer Services handled this very competently, or politely. It ought to have been a relatively easy situation to explain, and to ensure you were satisfied that the return was definitely made in time - and what you would need to do next.

Personally, I do think 30 days is a long time for a buyer to be allowed to keep an item -  and it is rather inconsiderate behaviour to leave it until the very last minute to start a return, unless it is due to genuinely unavoidable circumstances.