I've made a few trips to the hardware store and other errands over the years, using a backpack on my bike (years ago, when it was my only transportation, I hauled a small amount of groceries in a backpack and just made multiple trips) but this is the first trip I've made since I started trying to turn this mountain bike into a cargo bike. I've got the rear rack I talked about in that post, some basic secondhand fabric panniers secured with straps and zip-ties, and a milk crate from a consignment shop. I went for a pretty light grocery run for the first trip, just two totes of frozen stuff, and it rode just fine on the way home.

Thanks for all your advice! I think I'm going to upgrade to bucket panniers at some point soon, but I'm glad I can at least start using it like I'd planned.