Hi! I've seen your requirements n I'm interested in it, we have already developed most of the modules like shipment rate calculation based on location, payment modules, vendor based products etc.
@The extension need to apply a discount created in Promotions > Shopping Cart Price Rules if the shipping selected is 1, 2 or 3 method. also do not apply if 4 or 5 method is selected.
the total price will have a discount if only 1,2 or 3 shipment method is selected, is it correct? let me know if u r interested