Are iTunes ringtones a one-time purchase?

If you buy a ring tone from the Tone Store in iTunes, once you buy it, you own it for as long as you use the same Apple ID and it will remain in your tone library regardless of whether you get a new iPhone along the way.

When you participate and earn points, you get perks. It takes 500 points to be able to create your own Avatar here on the forum. Here are the levels --> Share what you know and get recognized - Apple Support

For your downloaded ringtone, Open Settings > Sounds & Haptics > tap any Ringtone and look for Download All Purchased Tones.