Xecrets Ez uses the same app for Free and Premium. Premium is activated with a license key; a long text string.
After purchase, open the license page from the customer portal or app menus. Copy the key, then paste it in the app or use the app menus to activate Premium. Repeat this when the key expires, usually once a year. The app will not fetch the license key automatically from the Internet for security reasons.
Actual payment is only done after the 10-day trial, unless you cancel. You then need to activate Premium again.