Add the ability to charge for items as a hire, where the total cost of a given item on an invoice is calculated as:
the amount of units x the duration of the hire (eg 4 days).

Currently I have to have the item unit field set to days, which allows me to invoice as a hire, but only for items I have only one in stock of. For items I have multiples of I have to list items with names like:
"1x item A"
"2x item A" etc.