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Not surprised PayPal didn't take the coupon into account, they'll just ask about the original total. Nor does eBay, I've noticed it'll say that the original total but you'll see the coupon deducted in your PayPal receipt/transaction details.
You'll have to call customer service to explain the situation and how they can reverse the transaction or how to proceed and pay the seller again, maybe give you a 10% coupon code to pay the seller again via PayPal.
To contact customer service:
Log on, click HELP on top menu, scroll down to click CONTACT US, and then scroll down to click CALL US.
Or private messaging via social media:
Facebook (US): https://www.facebook.com/paypal
Twitter (US): @AskPayPal