You don't have to pay again, unless you want a newer version of Pro
Your payment of $18.88 entitles you to free updates & email support for 6 months...whatever Pro installers you download in the 6 months are yours to use for as long as you like (it's up to you to save & back up installers). After the 6 months is up, you have the choice of keeping what you have or buying Pro again to recieve more updates.

The latest stable release is 4.12.11 & the latest Beta is 4.13.2.

Hope this clears things up for you