If you purchase a forum software package with Paypal, if you want your money back, Paypal won't help you.
A recent forum member here purchase IP.Nexus and wanted his money back and Paypal rejected the claim outright as the purchase was not a physical item, but was downloaded software.
Is Xenforo is purchasable via a Credit Card. Is IPB ? Is vBulletin ?
Might be a good idea to purchase with a Credit Card under certain circumstances.