I now have a Breezer for the grocery, clothes, library etc trips. I've been doing the car-free with a bike thing for a year now, and I had a very clear idea of what would make the bike easier for me to use. I needed a regular length bike that could handle a normal load (50 lbs) and would be easy for me to carry up and down stairs. Carrying trailers up and down stairs isn't fun.

If you're reasonably familiar with bikes, I'd look for a used bike for your commuter. A rigid frame is important. That means if you find you're carrying More Stuff, you can add a front rack. 42 cm chainstays are about the shortest I'd want. The Breezer has 45cm. A low bottom bracket also helps make carrying a load feel stable. Touring bikes will be somewhere in that range and will have a low bottom bracket. You will not get as much bike for $400 buying new, and in that range a lot of the bikes are too focused on being "sporty" to do much real work.

If you're grocery shopping for one or two people, most any bike can handle it. If you're feeding more or you like to buy in bulk, you'll want one that is better suited to hauling a load. If you've got 3 teenagers, a trailer might be called for. At 8 miles and with your schedule you will *not* want to shop every day.