If you don’t want to lug yourself and your luggages to Costco, there are luckily some other options. Yoyo Market is an online shop in English that sells products from Costco. Their selection is less than going to the store yourself, but it saves you the trip and it’s much more convenient. They will also pick up IKEA goods and ship it to you directly.
Another online store that sells American products is The Flying Pig. Their selection is a little bit different from Yoyo Market, and their website isn’t as sophisticated, but it’s an expat staple in Tokyo!