Thanks for starting on this. Yes I can see that it is going to be tricky.
However, lots of the Stroudco shoppers do hold a credit balance on their account and use this to pay for orders. I guess the way this will work is if a shopper is shopping on a particular hub, then any balance they hold with that hub can be used on their order. - they wont be able to use that credit balance to buy from any other hub or producer.
Maybe the balance could show up only when they click to shop with a particular hub. Would that be easier to set up than giving shoppers the option to be able to select which hub/producer they want to check their balance with?