It is not possible to add a surcharge to Stripe payments and only the balance payment will be deducted from a client's card. Please note: even if a charge is specified in the text box in your portal, it will be not be included in the Stripe transaction when the outstanding balance is taken and will not be billed to the customer.
Don't forget that as a Pitchup.com campsite you are automatically benefitting from a lower transaction rate with Stripe payments than the rates you would be offered as a stand-alone campsite/park. This saving, combined with higher customer bookings anticipated through the new direct payment option, should more than compensate for any lost supplement.