Alan,
I think you are beyond fair in only adding the extra cost of the hook. In retail, prices typically are based on wholesale plus margin. If you only charge the extra cost of the hooks, then you are not including any extra margin. Your customers should realize that, but I suppose some of them would not appreciate having your frugal virtue on their behalf pointed out to them. Pity that frugality and gratitude are no longer fashionable.

Ed