1. Go to Costco and purchase as many huge things in bulk as I can. People like to equate 'buying in bulk' with 'pricing', but honestly for me its just as much about the convenience: I don't care about saving $.50 on toilet paper so much as I care about having to buy toilet paper only once a year. I'm picky with some things (like hand soap), but like you said I know Kirkland products are going to be good enough.
2. For everything else, buy off Amazon/Target/TJ's.
One of the cues I'm surprised Costco hasn't taken is subscription-based shopping (as in, I set up a list of X items that I can schedule for a pickup every few months) that would make this process even better. Given the decidedly non-marketing-oriented layout of their stores, I don't think they'd miss the opportunity for up-selling.