Google Play is the primary App Store for Android. We offer aCalendar+ as paid app as well as single features as IAP (in-app-purchase) here.
Your license (both for the paid app and for IAPs) is valid with no time or device restriction. That means, as long as you are using the same Google account, you never need to purchase again.
Payment options vary by country, please see Google Play payment options for details.
For aCalendar+ bought as paid app after July 2, 2016, your purchase is eligable for the Google Play Family Library.
You can see the Google Play Refund Policy for details on refunds, but please always contact us if there is any problem (either from the help section of the app or through our support pages here).