Involve AliExpress. Sellers are notoriously shady, but AliExpress' own interest is to keep both sides honest. Unless you involve AliExpress formally through a refund request or claim, they are not monitoring your conversations. The seller made a mistake, I would expect they pay for return shipping.
Seraph is tan tan, have you considered ordering via email away from AliExpress? Velobuild, winow, sell on AliExpress but I'd much rather use email and invoices, it makes you less anonymous to them, becoming more of a customer, and less of a number. Also, AliExpress involves hefty fees, so it should cost you less to liaise by email. If they're showing you the same price, call their bluff.