OK, if I get what the OP is saying: the goal is to have a way of bringing an unexpected amount of groceries home. It sounds like you're using rear panniers on a daily basis. My move would be to use
Wald folding wire baskets on the rear rack instead. They're more convenient anyway (you can just throw your stuff in and then grab it out at your destination), but the other advantage is that they're really easy to tie or bungee stuff to. In my bookbag, I keep an extra bungee cord and a
Bagito micro-stuffable grocery bag. They come in handy really often, when I end up needing to carry more stuff than I expected.
My usual move when I end up with more groceries than I planned for: go to the produce department and ask for a cardboard fruit-shipping box. They're usually happy to give me one for free. Bungee it to the rear rack. Voilą.