My billing practice is to handle it as formally or as informally as you (the student or whoever else is paying) want. You can pay as you go or monthly, and I can e-mail or mail you invoices. If you need me to do something else (for example, billing your firm) just tell me what works for you.
For payment I accept Venmo (which is quite easy if you have an account; my username is doug-couchman), PayPal (ditto; username is my email address), inter-bank using Zelle, checks, and cash. I have never set things up to accept credit cards but if that’s important to you (maybe you want the airline miles?) I could do it, even though I don’t love the processing fee of about 3%.
I have never had to figure out whether I accept Bitcoin or any other cryptocurrency. I suppose I would do so if you want to pay me that way but you’d have to explain to me how to do it first.
I do accept cash, though this makes sense only for in-person tutoring. Please don’t send cash through the mail.
For more on my rates, go here.