Re-reading my post I see I unintentionally made Helpx sound flawless but I agree with other comments that you should definitely be cautious going into it, especially since you don’t have a lot of work experience, so you probably still need to gain a stronger idea of your own boundaries and what is acceptable in a workplace (both in general and to you specifically). I should have made that clear. Every host will be different. When you first start you should stay with some shorter term ones so you can see the vast differences between hosts early on, and get used to the idea that you can just leave if the situation is too messed up with a particular host. Keep in mind it is a travel site meant for people to have fun basically, and not actually meant to gain ridiculous amounts of unpaid labor from you even though some hosts will try to make you work a lot - which can be bad or it it can be “fine” depending on what you ultimately want out of the arrangement. Personally I have ended up putting some of my longer-term volunteering stints on there on my resume, and I probably could have gotten one of my hosts to write me a reference letter if I had thought about it. But the work at some of them was quite stressful. While at others it was relaxed and fun. And from a different one of these things like HelpX, I think it was WWOOF, I once met someone who said one of their previous WWOOF experiences felt like a cult and they had to leave. So, be cautious, and start with small stays, and leave if you are uncomfortable.

Signing up for a PO box is pretty easy. It’s going to cost somewhere around $20 a month probably and it looks like they make you start off with a 3-month payment. Read up on it here: https://www.usps.com/manage/po-boxes.htm - Because of the fee, I think you should hold off on getting one until you really know you will need one. When I got one I think I just walked into the post office and said “How can I set up a PO box?” and they helped me. I think they needed my ID and my proof of address (again, this often comes from some official mail of some kind but I don’t remember what I used, my ID might have been enough). Then you pay and they give you a key/combination number and you can come check your box any time because it’s usually in a separate part of the post office that is always open. If you get a big delivery like a box then you ask for it during normal hours at the counter.

I do not regularly get mail from any bank or government office

It’s worth it to see if you can sign up for the library with just your ID. Each library system will have its own slightly different requirements.

As for getting some “official” mail for address proving purposes, you could try applying to something like food stamps or health care or other assistance online, you’ll probably get some kind of letter from them even if you don’t get enrolled, and you can hang on to that letter with your name and address visible and that can likely work for this purpose. Edit: I forgot in your original post you mentioned you were in college at some point, you could also order a transcript to be mailed to you, maybe that would work too.