Add Quantity parameter to listings

Hi,

I would love to see Quantity parameter in DMS.

For example:

Seller by uploading could set a number of documents available for download, e.g. 100.
Then buyer could choose a certain quantity of that item and put it into cart and pay e.g. 4x £5 = £20
After purchase, the total document amount would decrease by the purchased amount. 100 - 4 = 96

Is it possible?