I'd go with one of the following options:

Get the Bug, and a Wald basket for the other side. That way you can put groceries in the basket and keep the other stuff in the Bug. You can also leave the basket attached to the bike all the time, so only one bag to carry when you lock your bike and leave it behind.

A cheaper solution might be to convert an existing backpack to a pannier. Arkel sells a kit that allows you to do this, but you'll have to figure out how to keep the straps out of the spokes.

Another solution is to put your backpack into a large pannier or basket.

Personally I'd consider Option 1 if you can afford the Bug. I have several pannier bags, but most of the time I just carry a single pannier. Usually it's my Arkel Utility Bag. I'm thinking about attached a Wald basket to the other side just so I can have the extra capacity when I need it.

Sean