I find it easier to do when I bike to Costco _without_ my trailer.

(I have two good sized panniers and can end up with ~$150 of foodstuffs packed well in them no problem. More often than not, I get less than that and add more stops to the trip to pick up from 3-4 places while out. And I get my exercise while I'm at it.)

When I lived closer to a Costco and had my membership, I used to do what I called cartless runs. Go into Costco, only buy what you can carry. Usually just the thing I needed and maybe one or two other things.