Just got the email today my cash payment made it to them. I did not send it registered mail because the lady at the post office said someone would have to sign for it and i was not sure if that was going to be an issue. Plus it was like $20 to send it registered so that was abput 25% of my small order. I put the cash in an envelope with my order forum then put the normal envelop inside a 6"x9" padded manila document envelop. I declared it to be documents on the paperwork for customes. Cost about $3 to ship. Took 8 days before i got an email letting me know my payment made it and that i would get another update when they were shipped.
Just my two cents, but I used https://transferwise.com/us/ on my order. Ordered Feb 10, dispatched Feb 11, and recieved them Feb 21. It was pretty simple like using Paypal, also one of the options for logging in...only charged me a $3 fee for the transfer. I too was not fond of sending cash through the mail, hope this helps.