Hello!
I 've written a piece recently about the plank, a technique I use the groceries by bike I have tried many methods (like a backpack) but this seems to be the easier and cheaper to use. If you are car free like me and you need to carry a big amount of groceries every week, this could be very useful for you.
You can have a look here:
https://bicycleobsession.wordpress.c.../20/the-plank/