It’s mentioned quite often in their financial filings but I don’t have an example handy and I don’t feel like digging through a 10K right now. However, here is an Esquire article that mentions it:

“Costco has stated that the highest margin they will take on a non-Costco brand is 13 percent and they strive to keep it closer to 10 percent.”

https://www.esquire.com/food-drink/drinks/a35749/costco-the-...