A feature that would allow you to create the mock-up for a proposal, then send it to the customer for them to input their sizes and quantities and then see their pricing and check-out. This is essetially the way customers check-out when using the designer. It would be nice to be able to use this type of interface for proposals as well.
Another adaptation of this that would be helpful is for customers who regularly place bulk re-orders. For instance uniforms. If they could choose the item they want to re-order, enter their sizes and quantities and see how their order volume affects their pricing, then check out once satisified.
This would avoid the customer having to use a site that's tailored for retail sales, or the back and forth with the customers that sometimes accompanies traditional proposals.