If I don't use a vpn, should I disable IPv6 to avoid my pc's mac address be guessed ?
While paying online, once I saw a bill that included my IPv6, should I be concerned at all?
No. You should enable privacy settings for IPv6, so that you use a pseudo generated IP, and not one based on the MAC of the computer. On Windows this appears to be the default, although I have never studied IP allocation on Windows in detail with IPv6.
IPv6 can indeed increase privacy through the use of ephemeral addresses that are short-lived.
As with all technology, you need to understand it and quantify the risks.