How hook into the end of an ecommerce transaction on a foreign domain
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I would like to integrate with certain vendors that sell ecommerce goods. What is the most universal approach to determine if a purchase has been made by a user on their site (ie. credit card / paypal entered g, etc)?

I would like to have the user briefly get redirected to my site after a purchase has been made (to enter a rebate code). How can I best (or have the vendor) hook into this pipeline right at the end of the transaction?

Any possible approach would be appreciated.