What is Product Pinger?
At each store, you often can sign up to 'notify me' email lists. Because of how email lists work, mails often get scheduled and batched to not overload email servers. This means that the product most likely is sold out long before you get the email from the store.

Product Pinger constantly check the stock on each store's website, and as soon as the product changes to 'in stock', you get notified - without any delay. This means that as long as you have your phone with you, you have a very good chance in getting the product before it is sold out.