As I mentioned in the description, the holder can be used on the bottom side while on the top the bungee is mounted to the battery holder. That way 1pc is enough. If it would be sold in pair, those who would need only one would be in trouble to buy a single piece.
I thought it can be confusing, that's why I put the reminder into the description to make it clear, if holder is required on both top and bottom, user must order two pieces.
Regarding the price of the adapter: I checked other 3D builders but didn't find any having similar support provided. The price is high as it's shared between Shapeways and me. If you are interested in more details, you can upload any design and check Shapeways fees applied.
Best regards, Janos