Here's the situation:

The client is a water bottling/delivery company, which is building a sideline in single serve coffees (k-cups). The zen cart is for the purpose of selling these coffee items. What I'd like to have is a shipping module to take into account existing water customers, who could simply enter their account number and have the coffee products delivered with their water.

I'm thinking it ought to be a fairly simple mod, since it really only needs to store one little bit of info, and wouldn't incur a charge. Ideally it wouldn't show up as a shipping option if the customer was outside the geographic area the water company delivers in (zip code based?).

I've run through the various shipping add-ons and I haven't seen anything that seems compatible with this idea. I'm not an accomplished php coder, but I know enough that I don't think writing this is outside the realm of possibility. I have absolutely no idea where to start though. Is there a shipping module that would be a good base to look at to start something like this?

I suppose the alternative is to tell them to use store pickup and leave their account number in the comments. All suggestions/comments are welcome.