Paying for proton with cash

I tried doing this for a different VPN provider based in the EU. I’m in the US. I was instructed (by Fedex) to fill out a “U.S. Customs Form” which requires full name of both parties, email (to speed up the process but it seems optional), full address for sender and receiver, and a detailed description of what is inside. I wasn’t given the option to send without a sender (return) address. I didn’t do it. That defeats the purpose of paying with cash. For what it’s worth, I have never been able to send anything using USPS, UPS, or Fedex in the U.S. without a return address and showing my government ID. Not sure how you folks are doing it. Are US folks just putting a piece of mail, no return address, in the post box with international stamps? Do you recommend doing the payments in increments or is sending $100+ going to get flagged by customs? I am fully prepared to lose whatever I send either way.