Modify WooCommerce 2.6 Invoice Plugin - Change Qty / UM

Hello, We have this Invoice Plugin - Smartbill and how it works: we have a button on order page and when we click it takes QTY and UM (for example qty: 1 and UM: piece) and goes to 3rd party website where it fills in the Client, Product, Price etc. including QTY and UM. What we want is to add another field/fields to some products so when we click the Invoice button it changes QTY and UM. For example: DEFAULT qty: 1 and UM: piece ... MODIFY qty: 7 and UM: bag. Thank you.