Quantity bought for bulk products after #974 and #1090 were closed

[serpico]

After the recent commits #974, #1090 I wanted to try to add prices for products in bulk, thinking I'd be able to add the quantity bought ( it's not a receipt, it's pics of price tag, maybe that's why it, apparently, fell through the cracks. ( @raphodn I remember you mentioning "You're still talking about the receipt ?" and I didn't really pick-up on it...)

Problem

There's no field to type up the quantity bought like 2356g, 5kg