The Booking Package WordPress plugin before 1.7.25 does not validate the payment amount server-side against the stored service price, deriving the expected charge from attacker-supplied request values instead, so an unauthenticated attacker can pay an arbitrary fraction of a service's real price.
https://wpscan.com/vulnerability/8df11bf9-1878-4220-b13c-82e8dadc3a08/