Thanks for making a donation!
I run the relay server, but the Debian maintainer agreed to bake an alternate hostname into the packaged versions (a CNAME for the same address that the upstream git code uses), so we could change it easily if the cost ever got to be a burden. It hasn't been a problem so far, it moves 10-15 TB per month, but shares a bandwidth pool with other servers I'm renting anyways, so I've only ever had to pay an overage charge once. And TBH if someone made a donation to me, I'd just send it off to Debian anyways.
Every once in a while, somebody moves half a terabyte through it, and then I think I should either move to a slower-but-flat-rate provider, or implement some better rate-limiting code, or finally implement the protocol extension where clients state up front how much data they're going to transfer, and the server can say no. But so far it's never climbed the priority ranking high enough to take action on.
Thanks for using magic wormhole!
We don't have that yet, but the two sides attempt direct connections first (to all the private addresses they can find, which will include a public address if they aren't behind NAT). They both wait a couple of seconds before trying the relay, and the first successful negotiation wins, so in most cases it will use a direct connection if at all possible.