The paid app fee is a one-time payment. It's not monthly or annual. There's no time limit; you can use it as long as you don't delete it. Google Play offers a refund if you have a download problem. To pay from your account, your account must be enabled for purchases. Otherwise, your payment will be declined.
The app covers all 4th-grade math topics. There are fewer tests, and most activities are designed to be performed by entering results with the keyboard. Natural numbers, addition, subtraction, multiplication, fractions, geometry, and measurement conversions (time, length, liquid, weight) are unlimited. Video solutions are available right next to the topics. For addition and multiplication problems, and questions with shapes, you can draw with your finger and delete with the delete key.