payments from customers

Hello,  I have a customer who wanted to order 4 different sets of cards from my website, but when she clicked on "add to cart" for all 4 sets, the final cart price shows shipping times 4 instead of 1 shipping charge.  How can I fix this?   I notice if she ordered 2 or more of the same set, it does show only 1 shipping charge, but not if she orders different sets.  Thanks for any help.