Proton accepts bitcoin, but i use trocador to pay them with a monero to btc swap where the output btc address is the one proton provides me to scan or coppy
Snail mail? Make sure to use a security envelope to prevent someone from reading your mail by spraying computer duster (when sprayed upside down it reveals the inside of a normal envelope, then evaporates). You can also put a few grains of kool-aid powder in the envelope seal to indicate if the envelope was forced open with steam.