Our system can only decrement your Inventory stock counts after receiving confirmation from the payment processor of a completed payment for X number of units. Otherwise, if we counted items added to any cart against your inventory stock count before checkout, then all your available stock would get tied up in pending cart orders that may never even proceed to checkout.
E.g., suppose you have Inventory Control set for a product with 5 units left in stock. Buyer A adds 3 units of that item to their cart. Buyer B can also add 3 items to their cart, because Buyer A has not actually purchased anything yet and may not ever do so (buyers "window shop" all the time by adding things to a cart and then abandoning their cart without buying during that visit). When Buyer A takes their 3-unit order to checkout, if their payment is completed before Buyer B proceeds to checkout, then Buyer B would see the order quantity in their cart automatically revised down to 2 units; however, if Buyer A's payment is NOT completed before Buyer B also takes their 3-unit order to checkout, then both orders would be placed for 3 units, resulting in an Inventory count of -1 after we receive those payment confirmations, and subsequent orders for that item would not go through, due to being Out Of Stock.
