Suggestion: Allow manual addition of UPC code

I have items (e.g., puzzles) that do not have ISBN numbers. There is a space for UPC code (yay!) but I cannot add the UPC. Please allow this.

I think this is a major flaw not having the ability to enter ANY identifiers. You have to have a source that has that number. Problem is sources don't put in all the different identifiers. They may just use one, ie:Amazon books