O.P.
You may want to seriously consider A SINGLE, 'Nashbar Townie' pannier (or other open top 'shopping bag' style pannier) as an alternative to the somewhat expensive 'touring centric' panniers you've been looking at.

I've been using 'Townies' since shortly after purchasing the 'Heavy Hauler' new. My experience has been extremely positive using them in exactly the way you describe. Caveat: I would consider the 'Townie' a light duty pannier which works best with loads BELOW 15-20 lbs, other/more expensive shopping panniers have better load ratings. Perfect for resupply-day trips to work.

With the addition of the 'Sporty-Quick' bike, I often find myself taking a pair of folded up 'Townies'' in my 'Mountainsmith' lightweight 'overnighter backpack', for the quick trip to the market. Loading the heavier items into the shopping panniers, the lightweight/overflow into the backpack if necessary.