Shipping prices are fixed per order. The prices should cover the minimal cost of you handling the order and shipping a small parcel (i.e. a few prints). Your actual shipping prices may be much higher when bulky items like framed prints or coffee mugs are ordered. To account for those higher per-item shipping charges, simply set the base per-item cost high enough to handle both the physical item cost and the additional cost of shipping each item.
Example: actual product and shipping costs
Use the final column, Per-Item Total Cost, in your price plan to account for the extra shipping cost associated with bulky items. Using this method prevents the customer receiving a large shipping charge toward the end of an order.
See also: By-Weight Shipping Charges