Bicycle trailer that can secure cargo? How do you secure your prior purchases in your trailer when you go into another store to make more purchases?
Unlike in a comment above, I'm not going to assume that you can carry the rest of your shopping when you go in the second shop. But hopefully you can carry any valuable items, then leave cheap bulky stuff in the trailer.

I do this with panniers and backpack, buying the more stealable items first and keeping them with me, then buying fruit and veg, leaving them in an open pannier, then going to the supermarket for the rest of the food shopping. The open pannier is on the basis that a closed pannier could be full of something more tempting than a bag of onions.