Invoice Items at a loss - Client price should be allowed to be less than unit price

I paid $42 for a small item for my client's project that I don't want to bill them for. I would like to keep track of the expense by putting a line in an invoice that shows my cost but zero for client price. Design files won't allow me to enter it that way (client price must be higher than unit price it says). I think this is a flaw - surely there are times when for whatever reason you take a loss on one item.