I have noticed on my past FAA multiple print orders,....after the minimum shipping is added for one item,....additional items shipping cost decreases.
I do not think it is a discount,....but rather a reduced overall bulk shipping cost because mounted prints are multiple packed securely in either a very large box or as few boxes as possible.
This amounts to a lesser shipping cost for you when you order.....much cheaper than boxing each and every piece by itself.
By the way their packaging is very well done assuring none of your enclosed merchandise gets damaged.