Add items from views to another form

Add items from views to another form

I've created an example app to show my question: you can find it here

I have two form:
1. Inventory : where are stored all items
2. Supply Ordered : where are stored all orders made for new supply

What I'm trying to obtain is simple: once a supply which is in the Supply Ordered form is delivered I wish to add its quantity directly into the Invetory without the need to delete it from Supply Ordered View and then insert manually into the Supply Ordered View.

I've tried with a Add Record action on the Add-->On Success event but even if field name it's configured not to accept duplicates I can only obtain that two objects, with same name but different quantities, are added.

Moreover, I can't figure out how to properly configure the "Add Quantity" action: i've tried with Fetch Records and than Update but I can not define a new quantity equal to the sum of quantity+quantity_ordered.

Many thanks in advance to everybody who might have five minutes to spend on this helping me through this problem! ( oh, and please beg my poor english ;) )