I have Mullvad separately. It's a bit annoying that you can't simply connect an already paid-for Mullvad account to Tailscale. (But then, connecting a VPN account to a Tailscale account will remove any anonymity from the VPN account you've just bought anyway).
I like the mechanism of buying a sealed voucher from Amazon - so Amazon knows I've bought a Mullvad account but doesn't know which one; and Mullvad knows someone bought an account on Amazon but doesn't have any of my details. There's nothing - except sometimes my originating IP address, perhaps - connecting me to my Mullvad account.