Well, having Paypal is one thing! But did you turn it into a business account or know how to make an Invoice?
Most of the time you're going to find yourself with the possibility of being scammed so to protect both yourself and the client (when genuine) it is important to
a. Have a business account
b. Not allow payments via "Friends and Family" or "Pay for goods and services", for F&F there is actually the thing that people cannot get refunds for this meanwhile for PG&S while there is the chance of refund there isn't actually much support for an artist if the client actually does a refund even when they get the product and accordinly to what they originally wanted
Making an invoice not only gives you the chance that while creating it, you can remove your personal information such as phone number, email and address to protect yourself, and you can detail the commission's stages of process to guarantee that in case of a refund the client actually accepted the product, received it in time and form and such. A common scam is to claim the product is not what they wanted, or that it didn't arrive accordingly to the artist's TAT.
It is important that you make an invoice instead of giving your Paypal email to the client, since it has been more common to get scammed this way, there is this current one called "Draw my child's dog for his birthday, I'll pay you $600" or this other one of "I have a business where I connect artists and clients together, we get 50/50", while you're focusing on Music, the risks are still the same, so be careful
Another way to avoid scams is to make a Commission Form or setting up Ko-Fi Commissions
Well, this is going to be more complicated, it'll be a matter of where you live, your currency, if there is inflation in your country and USD actually have a different worth and such.
A lot of people will tell you to charge accordinly to the USA or European pricing, by hour, or depending where you're trying to sell there is going to be a minimum price for a specific type of price.
Pricing could be complicated since there is a lot of things to keep in mind. First your prestige, your experience, amount of clients willing to pay you, time, etc, etc.
When starting, while everyone will tell you to charge what would seem worth for you, I rather recommend you to compare yourself with people around your skill level, see what are the prices for starters depending on quality and turn around times. It may or may not be a bit underpaid but you gotta start somewhere, then as you gain a bit more of popularity, regular clients and such, you can slowly start to increase the price.
I don't recommend much providing different prices for the same work to different clients, the least that you want is for them to find out, some will be understanding but the mayority would feel that you "scammed" them by charging them just a tiny bit higher, even if the price was put by the other client and not you.
Is not a bad thing, still remember that the chance of the client not paying the rest of the commission is a possibility so never offer high-quality music, always small preview and with a "watermark" which is like a quote being repeated, for example "ar-ninetysix commission". Make the client clear that they'll only receive the complete song without the annoying voice in repeat once the payment is complete .
Well, it is important to have Terms and Conditions, from the rights you'll be giving the client about the piece they'll receive (Still, once it is in their posession they can do whatever they want so try to not be inflexible about the possibility of them doing something with it later, for example, using the music in more than one place, burn it in a disk and sell it and so on. You can put a rule but to be honest if they follow it is beyond your control)
Turn around Time, basically how long does it take you to make a piece. If you accept refunds, when you're giving the high-res full song, where you're giving it be it an email or drive folder. If you accept or not extra money to finish the commission faster and let clients "jump in the line" and so on. I know is frustrating but it's a possibility
If you're not sure about what to put on your ToS or Instructions to Buyer, you can always try to find other artists who make music and research their Commission Info.