Add a "donations" field to the payment process
We like to give customers the option to donate £1-£5 to the AONB who look after the beutiful landsacpe that surrounds out site and upon which many of of our campers stroll. A simple dropdown box would suffice. Clearly we'd ask you to allow for this post commission calculation!
Generally we try to remove fields from the booking form to prevent customers abandoning the booking, but this might work on the confirmation page.