Portion of sale price as a commission to a collaborator

[Alan15]

Anyone recommend an app, or offer advice, to allow a customer to be able to select a third party who will receive a percentage of a sale.

A customer is making a purchase, they are asked to select a collaborator who will receive a percentage of the price of their purchase.
Apps like CollabPay work great except they are based on products connected to a collaborator. I need a customer's purchases to be connected to a collaborator.  Probably a 'donation' type app would be similar.

Thanks