Coupon not included in purchase

[adanament]

I have purchased an item that has a discount coupon.

I was expecting a screen to place the code in for the discount which would then be reflected in the final purchase.

This did not happen........ went straight to being paid for full amount.

Do I contact the seller or does Paypal have a way of correcting the transaction or do I cancel the order then re buy with redemption code.