Zapier is a web application that integrates with ONTRAPORT to add or update Contacts. In addition to adding the Contact to ONTRAPORT, Zapier can also add a Tag to the Contact.
Set Zapier to tag the contact based on the purchase. Create a global Rule in ONTRAPORT that looks for that tag to be added, and then uses the action "Add Product to Purchase History". In this example I have a product named "Coffee" and a Tag named "Customer: Coffee". The following global Rule is created in Contacts > Rules:
The Tag "Customer: Coffee" is added to my contact Isaac Newton, and the Rule runs and adds the product purchase to his history:
Note that the Rule uses the price set for the product in Sales > Products.
The sale will be reflected in sales reports, Dashboard metrics, and will otherwise act like a regular purchase through an ONTRAPORT order form, including reporting Partner Program commissions, etc.