Understand the stock management process deeply for fast selling items.
All stock that is available, can be added to the carts of multiple concurrent users.
The number of items in everyones open carts can exceed the total number of items available.
When items in shopping carts are purchased, everyones carts may be trimmed down to the remaining quantity.
The first one(s) to submit their order at checkout (after entering their details) are the one(s) who gets the remaining available stock.
At the very last moment prior to submitted an order, stock levels are once again checked.
Payment is irrelevant in the stock counting process.
When an order is cancelled (manually / automatically), then stock is returned to available status.
Payment is not relevant in the above process. If an order is not paid, or is paid via online banking for example, there could be a period of 10 minutes or a whole business day before the payment status is confirmed. Therefore, a submitted but unpaid order will lock stock. If you do not want this to happen, you need to cancel your unpaid orders after a selected period of time, that allows your customers to actually pay you before you cancel the order.
Some believe that the stock should be locked as soon as you add the item to your cart. However, there are so many window shoppers and robots out there, that if we took that approach, then all the stock would be allocated, and not availble to others, until a certain time had elapsed when the open cart would get deleted... We are very generous with our open carts, allowing users to come back several hours later and continue with their purcase, assuming stock is still available.
You can specify settings in the shopping setup area to automatically cancel orders that have not been paid. Please use at your own risk, if you are taking payments that may require manual reconcilation within a limited timeframe.