Normally the way this works is that XYZ site offering ring tones forces a would be customer to sign up with their payment information and either A) Register their cell phone number or B) Gives them a user name and password that they can use to log into the service from their phone.
The website would track any downloads and charge accordingly. For example, if I jump onto Sprint's website (my cellphone provider) I can either purchase ringtones directly from their site and download them to my phone later or I can log on to their site using my phone. In both cases they know who I am and apply the charges to my monthly bill. Other service providers work the same or about the same way.
Also, there are (at least) 4 different formats that cell phones can play and they differ from service provider to service provider:
For example, my cellphone (Sanyo Katana) uses the qcp format for ringtones. Check out this forum post about 3/4ths of the way down a guy describes how he gets ring tones on his cell phone and you can kind of get an idea of what the scope of this project is going to be:
Lastly, as chronicle pointed out, unless you use some sort of Server Side language or technology(PHP, ASP, ASP.NET, Ruby, etc) you will not have the ability to process PayPal accounts or any other form of payment for that matter.
Read this if you want to know how to get a correct reply for your question:
.: Wrox Technical Editor :.
Wrox Books 24 x 7