There are two main reasons why there is a 90-day period in order a purchase to be considered as valid.
The first reason is that for shared web hosting plans, there is a 90-day money back guarantee. This practically means that the end customer might have requested the cancellation of the plan.
The second reason is that the affiliate department checks if the hosting plan is used by the customer. There are some checkpoints to validate this, e.g. if there is a domain name linked with the web hosting plan, if the nameservers have been updated, if the customer has started building a website, etc. That is why we provide a reasonable time period for the end user to have started working on his hosting plan. The sales validity check is always done at the beginning of each month, where all sales from the previous month that are older than 90 days are checked.