How to understand that the user bought the application?

Question

Google Play

Hello.

The previous version of the question. Crooked wrote

Хочу выложить приложение на продажу. При покупке приложения пользователь (дополнительно) получает оффлайн услугу. И вот вопрос, как мне понять что пользователь, который написал мне на почту купил приложение? есть какой-то идентификатор транзакции покупки - который виден продавцу и пользователю или список купивших пользователей?
Есть вариант сделать в приложении, что-то типа кнопки "заказ" - но это крайний вариант. Пока не хочу его использовать.
В гугле нигде не фигурирует, что пользователь получает бесплатные доп. услуги при покупке приложения. Это как временная акция.

There is an application in 2 versions. Free and paid. The paid version has additional functionality. Also, during the promotion period, we want the user who bought the paid version to provide additional services in offline mode, i.e. if the user has bought - he can contact us by e-mail and receive services.
Question. How to understand that the user who wrote to the mail really bought the application? - Are there any purchase identification parameters available to both the user and the merchant?
There is a solution - add an "order" button to a paid application - and everything will work out in it. But these are extra costs.

Answer 1

You can use a friend's mail in a paid application, when the user pays, he gets a mail for communication that is not available in the free version, and you can also make an identifier in the paid version, when letters arrive, the ID will be displayed in the letter itself, which will indicate where it came from application from paid or free

Answer 2

Any online store or billing with a personal account.

Answer 3

Post two apps, one free, one paid.