Not 100% related to the OP, but we recently realized that we each have a different number on our Costco membership card, even though they are in the same account.
Figured this out because only receipts for purchases made with your membership card show up in your recipes tab in the phone app.
This actually turns out to be nice because we go to Costco together but buy stuff separately. So only the stuff I bought shows up on Costco app on my phone, and the same goes for her.
P.S. The receipts being available in the app is a godsend for me. Unlike the grocery store, which is 99% groceries for example. I can be buying anything from a computer to a microwave to steak at Costco and I like to break out those categories in my financial (quicken-like) program. I'm rarely near the Costco receipt when I'm categorizing new purchases, but typically have my phone with me.