On bandwidth abuse, Fly may refund within reason: How concerned should I be about a surprise bill? - #9 by kurt
To limit bandwidth, you can employ tc (I haven’t used it on Fly, so you’ll need to test if it works as expected): Introduction to Linux Traffic Control
Tailscale engs mentioned that they shape traffic on their relay servers hosted on Fly, but I’m not sure what they use. May be someone at Fly does.
Surprising it works. Docs are open, any one can propose edits: https://github.com/superfly/docs/blob/d52feb7fa83c4dbf9589742c4f2ae2fdc6e16338/app-guides/udp-and-tcp.html.md.erb