The App Store version of Viper FTP has two types of licenses - a lifetime license paid once and a subscription license paid monthly or annually.
The license can be purchased and managed from the About window:
From Viper FTP menu select "Viper FTP" -> "About Viper FTP":
Viper FTP App Store license
Viper FTP icon

Viper FTP User Guide

The About window displays the license type, license validity period, "manage/start subscription" button and "restore purchase" button:
Use the "Restore Purchase" button if the license has already been paid for and the license is not visible (eg after downloading Viper FTP to another Mac).
Monthly and yearly subscriptions include free trial period (7 days). You will not be charged during the trial period. Subscriptions can by canceled any time from your App Store account or by pressing "Manage Subscriptions":