People go to Costco and only get a couple of things and not a cart load? You've got more self control than I.

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.)