My suggestion is to have an automatic calculation of the Price you have an item
for sale minus the discount show up non-editable somewhere on the screen (probably
next to the discount).
This would allow for a quick view of what the item is selling for with the discount
placed on it.. It would solve the problem of going below cost when trying to
just make your money back on items.
I know BrickShop and those will do that but I personally don't use those as I
haven't figured out a good method of integration.