Check the Razer store for stock of the product you want at a specified interval
Select your country, enter the product ID, set the checking interval, decide whether to notify for out of stock items, and go!
The product ID can be found by navigating to the page on the Razer Store, opening the source (F12 on Chrome), and navigating through the collapsable sections until finding a seciton that looks like:
ecomm_prodid: "309149100"
This is the product ID, copy and paste it into Razer Notifier!
Sometimes an item will report as out of stock to Razer Notifier, even if it's not (usually when it says "Limited items/quantity" in the Razer Store). The out of stock checking option exists as a reminder to go check for yourself if the item is in stock, even if it reports that it's not.