Says purchase code invalid (bought today)

I would like to posts else in the forum, but it says my purchase code is invalid so I have to post here. I bought the plugin today so the code must be valid.

I have the following question;
I want to track the price difference between an old and new price. So if price gets updated, compare with last price.
I can do that easily in php. I would like to know, what action hook should I use to not interfere with the plugin?