You need to process the order as described in the general terms and conditions that you have submitted to beslist.nl. This is probably exactly the same as the handling of an order that had taken place directly at your webshop.
Within the Beslist shopping cart model the risk of cancellation or return is the responsibility of the webshop. If this leads to unreasonable or extreme situations, you can consult our costomer service regarding the possibility to (partially) reimburse the deducted commission or transaction costs. email@example.com