The panniers don't have to do all the protecting of the cargo.

Pad the laptop in it's own carry case. I got a laptop case, for not much---I wanna say less than twenty bucks, that has a shoulder strap/briefcase handle outer shell with pockets and an inner sleeve that is padded with just a Velcro closed flap. I use the just the sleeve a lot more than the whole case.

I have a pair of permanent, joined-together panniers. As handy as they are I wish I had the option to only have one. Most of the times the pair aren't near full.

I'm thinking of getting one of the "grocery" panniers. Easy off with carry handles. Perfect for most store runs.