Simple math question

Simple math question:

Customer ordered 2 cases with 10 packs of an item in each costing $17.05 a case. They received the wrong item with 8 packs in a case but were still charged $17.05. What is the amount that should be credited?

Please double check my math 🥺: 17.05/8= 2.13*2= $4.26 credit amount