For both Synergy and Synergy Ultimate, it's a one-time payment with a maintenance license included, which can be extended optionally. The maintenance license gives you technical support and software updates.
After your maintenance license expires, you will continue to have unlimited lifetime download access to the version of Synergy you purchased, so there is no obligation to extend your maintenance license. In other words, if you decide not to extend your technical support and software updates, you'll still be able to use Synergy and you'll still have access to the download that you paid for, forever.
Optionally, if you want to, you can purchase an extended maintenance license for $9.00 USD per year. Both personal editions of Synergy include a maintenance license, which gives you access to software updates and technical support. Depending on the edition you purchase, Synergy includes either a 1 year or 5 year maintenance license as standard.
This does not apply to Synergy Business, which is an annual subscription.
If you have any questions about paying for Synergy, please let us know.