RE: Coupons in Market Press. Coupons should be more flexible. They should be able to be used for a single product or a group of products, category(s), volume discounts, start and end dates, and number of uses. Market Press only does start-end-number of uses.
None of the wordpress shopping carts I have tested do coupons with any flexiblity. Only on the php stand alone carts and they are notoriously insecure.
I hope that MarketPress can be modified to add my suggestions above.