In order to add to products_quantity, is it simply '" . zen_db_input($v_products_quantity + p.products_quantity) . "'
All 3 suggestions - my 1 and your 2 all overwrite the products_quantity, testing each a couple times to make sure.
What is missing?
At my line 1551, I see that there is a query to get the record's current contents which selects p.products_quantity as v_products_quantity - so it should be drawing current qty and adding the incoming v_ quantity.