Seems a bit difficult, I'm not particularly interested in going through all the hoops that come with trying to own cryptocurrency anonymously, and actual cash payments don't seem practical considering that the Signal organisation is US-based (I don't live in America; I'd absolutely try my luck with sending a letter with a 5€ note to e.g. Switzerland).
And it's not really about the price, I'd happily donate 3.50, but not directly tied to my messenger account.