The OP's daughter has already chosen, but I hope he won't mind if I continue the discussion on a tangent.

I have a Topeak MTX trunk bag, the one with the fold-down panniers. I just love it, but sometimes, it's a bit small. (each side will hold just a 5 qt. engine oil jug)

I'm THINKING about buying a set of panniers, but they would only get occasional use, so I definitely can't justify new Ortleib. Amazon has a few highly-reviewed brands for 1/3 the price, so I'm thinking of going that way.

I could also just wear my big backpack for the occasional larger grocery run.