For this I would find it easier to add some simple PHP/MySQL but that just me.
1) You need to make sure the unique id get posted to the payment gateway and is also returned.
2) When the user arrives back at your site usually the "thank you order complete" you can add the code needed to that page.
3) Make a record set to filter the order from the orders db via the transaction Id sent back from the gateway, not the id for each item purchased.
4) Depending on how many matches or rows come back start a loop limited by the number of matches.
5) Inside the loop insert a update transaction eg..
($stock = ($stock - $purchased_amount))
$prod_unique_ref' = "#record set result for unique product id"
$updateSQL = sprintf("UPDATE products SET STOCK_LEVEL='$stock' WHERE Ref='$prod_unique_ref'");
6) This will go thru each item one by one and update the amount remaining.
There are more efficent ways of doing this but for just a simple update it does the job and it is easy to make and understand