The reason you may receive an invoice after your subscription has ended is that there may still be sales that have not been invoiced yet.
This could happen because your subscription has a longer payment process, resulting in you being billed for a sale or conversion even after your subscription has been terminated. For example, if your subscription has an approval time of 1 month, it means you will receive an invoice one month after your influencer has generated a commission through your collaboration.
Another typical reason could be sales coming through because of the cookie time, which results in you experiencing ongoing sales after the end of your campaign. This is why you might experience being charged for a sale. The cookie duration lasts up to 30 days.
If you want to see which sales are associated with a specific invoice, you can read here on how to find that information.