Add a coupon during checkout based on email

[ran_alexander]

Hi

due to an affiliate situation we need to discount customers based on their email address.

A customer enters his/her email, then we need to check it against a webservice, and add a discount to the checkout.

As I cannot change the checkout form to do this via JS ( or can I? ) I guess i have to hook into some API.

What API can I use for that use case?

As the email lookup table cannot be prepublished to shopify (like auto-discount-for-email...) we need to call the service during checkout.

Any Ideas?