If you are paying with PayPal, simply do not use the "PayPal Express Checkout" button, because what that button does, is to use the PayPal login/password data in order to SAVE you from entering all your details on Esellerate (including the email), but of course, it would defeat the purpose of the Coupon, since you must insert the email from the previous order on Esellerate, in order to activate it.
So, if you use PayPal, use the regular checkout button, instead of the PayPal "Express" checkout, so you'll be able to insert all your data in Esellerate (including the *original* email used for the JFK V1 order AND the Coupon code ), and THEN select PayPal as payment.