The usage and invoice for my enterprise account indicates an amount for 1 "Additional Team Seat," but I only have one seat, which is me. After much time sunk inspecting and searching, it seems that 0 seats are included with Enterprise, meaning my 1 seat should be billed.

However, the terminology "Additional Team Seat" is very unclear and misleading here. Since I am the only user, and only 1 seat is being used, the term should probably just be "Team Seat(s)." "Additional" implies a non-zero number of seats are included, and they are not.