[NSFW] Event Booking plus a offer to join Foundation – booking limit issue

We have added an option to join our Foundation when a person is booking a spot for one of our events
The problem is the limit for the number of attendees for the event and the limit for membersips are being added together and showing there are more spots available than actually are
On viewing the event post back end the attendee limit is correct , without the number of memberships available being added to it
Does anyone know how to stop it from adding adding these two limits together, as they are separate

The page I need help with: [log in to see the link]