A product page can load perfectly while the product is out of stock or no longer sold. Product availability checks look at the product itself and email you when it becomes unavailable, with a similar replacement already suggested.
What's checked. Amazon product links on the US, UK, Canada, Germany, France, Italy, Spain, Japan, India, Mexico and Brazil marketplaces. Links to other sites, other Amazon marketplaces and digital items such as Kindle books aren't checked.
Choosing which links are monitored. Each plan includes a number of monitored links and a check frequency (see plan comparison). On plans with daily checks for your most-clicked products, those are re-ranked every day by clicks, so the products that earn you the most are watched most closely and the rest are checked weekly. Pick your links under Monitoring in the account menu, on the Product availability tab, or from any link's targeting page.
When you switch links. Switching a link on or off never triggers an instant check. Every link has a fixed check time, shown as Next check, which keeps things fair for everyone, and a link can't be re-checked sooner than its normal interval just by toggling it.
How alerts work. To avoid false alarms, a product must look unavailable on two checks in a row before you're emailed, once. After that it's checked again until it's back in stock. If a product's availability can't be determined, no alert is sent.
Monthly check allowance. Each plan includes a monthly allowance of checks that comfortably covers its scheduled checks. It exists to prevent abuse and you're unlikely to ever see it.