Thanks Chris for your answer but the folks at http://www.arvixe.com/ who do the installation on behalf of their customers told me I do need to purchase the paid membership feature separately from xenforo in addition to the basic license. If I need them to do the installation for me who should I pay for the license? Them and they pay xenforo or xenforo and then give them some info that I get from xenforo to do the installation ?
We have a User Upgrades system built in which allows you to create upgrades that your members can purchase for a one off fee or continuous subscription. After purchasing the user will e promoted to a user group of your choosing.
If we're talking about the same thing, then that is built in with no extra cost.
For any transactions we would only ever want to deal with license holder (you) and never with a third party.