I need to make a decision: I pre-pack the cards? Or the customer picks their own selection?
Research into this (my Google search) brings up a lot of picking your own fruit and vegetables, but this is actually interesting as it influences how people will make decisions about other products. If they are acting one way with their groceries and are being advised against waste in this sense, then maybe they will apply this theory to other areas in their lives:
Looking into ‘pick your own’:
This is a lovely website called Cable and Cotton where customers can choose their own lighting by selecting the colours and cable lengths.