We are so sorry to tell you: Out of Milk Pro is not currently available for new purchase right now.
If you bought the Pro version - you can still use it like before. You should not see any ads, as long as you are still using an Android device. Your Pro subscription should follow you to any new Android device as well - it's linked to your account. If you have any problems with this, please contact us at firstname.lastname@example.org.
Sadly you can't buy Pro at the moment as a new user. While we are working on solutions that will allow us to offer Pro again in the future, for now it isn't possible. Ads are currently what make it possible for us to offer the entire app, with all functionality, to all our users, free of charge.