How to specify fields to transfer?

How do I specify which fields to transfer to Quickbooks? Only the email address is coming through for customers, and we're getting no orders transferred at all.

Also, I need a way to send orders/customers that are not in more states than just "Completed". It may make sense for us to also import "Authorized" orders.

Should there be a simple interface set up to include/exclude which fields of the Customer and Order objects get transferred?

I'll probably have to develop a lot of this myself if it's not already configurable, and would like to submit my work as a patch if possible.

The client has already tried other solutions (most notably, Courier) and they have failed to fulfill their requirements, so this WILL be developed.