I also thought that Tailscale would probably incur some type of charges after using it that much, though Im not super familiar with their free tier policies and how sustainable they are in the long-term.
Their free tier is pretty generous because it's basically a way for Tailscale to get homelabbers hooked on the product so they'll recommend a corporate plan at work. They even state as much: https://tailscale.com/blog/free-plan
The Pi 3 was essentially free to me because I already had it on a shelf. When I duplicate this setup at some other relatives' homes, I'm planning on using an Orange Pi Zero 3 ($30 CAD, quad core A53, gig of RAM, gigabit Ethernet).