If they had listed a line item for tariff fees then I could see the argument and would say that any refunds should go to customers. By not listing a tariff line item, Costco absorbed the additional costs and likely increased prices. In that scenario they, Costco, are the ones that should be entitled to a refund.
This is the same if you walk the chain backwards. Suppliers to Costco that simply raised prices and internally absorbed the tariffs are the ones due a refund, not Costco. Suppliers that sent Costco and invoice with a tariff line item should be on the hook to refund Costco (which means they should be seeking a refund from the US)
Amazon did try to add that line item and the administration pressured them to remove it. And you are making a very big assumption that either Costco or their suppliers absorbed the cost of the tariffs. Because I don't have a link handy, one study I read said more than 80% of the cost of tariffs came from the consumer's pocket, not the supply chain.
When I say absorbed I don't mean they didn't raise prices. I mean they didn't just transparently pass those extra tariff fees on to customers as a line item. With that increase in base costs they either lower their margins and earnings or they increase prices to keep them in a stable range. They are a for-profit business so it's highly unlikely they'd simply absorb the increased costs and not raise prices. There may have been some initial elasticity where costs rose faster than price rose.
This is the same if you walk the chain backwards. Suppliers to Costco that simply raised prices and internally absorbed the tariffs are the ones due a refund, not Costco. Suppliers that sent Costco and invoice with a tariff line item should be on the hook to refund Costco (which means they should be seeking a refund from the US)