I use PAC Ultimate (one size down from the O.S. Ultimate) to buy groceries just two days ago. The size of the bag expands along with expansion straps that allow me to close the bag and ride with no issue.

Groceries, a large backpack style with roll top or large sized messenger bag by most makers would work. The Pac bag I have also has a second X-strap to distribute weight over two shoulders but I seldom need to use it.