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 them. 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 (also easy; my username is my email address), inter-bank using Zelle, and personal and corporate checks. 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 made sense only for in-person tutoring (and never really was comfortable for me even then). Please don’t send cash through the mail.
For more on my rates, go here.