It's not a bad idea. In addition to the convenience of not driving to the store each week, you also get to choose what food the mice eat, which means a chance to feed your BP's extra healthy, well-fed mice.

You could probably get away with buying two mice that are about the right size or a tiny bit smallish...feed one "today" and the next a week later. And pick two other mice that are a bit smaller...and save them for the next week or two.

Just one major thing to be careful of: Make sure you get all the mice the same sex and that none are preggo when you buy them. That could be a hassle you don't want to deal with.