I'm fairly sure it's only a one-off payment.
Even though it's only 99p though, have you considered making your own ring tone - free and much more fun :)
I've done several for my phone.
Download a programme called audacity - available here:
https://audacity.en.softonic.com/download
This programme allows you to take a selected part of a song and save it as a separate file, so as long as you have the song on your laptop, you can crop it; then you connect your phone to your laptop; transfer the file into your music folder on your phone and then you can use it for your ringtone.