Price Adjustment/Discount/Upcharge question

Good afternoon. Please advise me on the question below.

Customer order our import product in either the 20-foot container size or the 40-foot container size. The 20-foot container is less cost-effective. . .a lot of space on a boat, but not as much product. 

We are looking to encourage customers to move to 40-foot containers by adding a small upcharge on the 20-foot order. For example, if the base price of a product is $100, we will upcharge to $110 if they order enough for only a 20-foot container, but they will pay the "normal" $100 if they order the larger size

How can I set up an adjustment that will kick in when the container size is 20? I assume I will have to do a customization to put "Trailer20" or something like that on a field, so that the adjustment can fire.