I forgot to apply coupon,,please refund the difference of coupon amount ?

The system does not allow you to apply the coupon after purchase.

And we can't apply coupon when you forgot to use.

The reason is ,,,

Our coupon usually has the restriction of "Only once per person" .

We have to "refund" the coupon amount if we meet your request, 

in this case, the customer can still use the coupon for the next purchase,

because the coupon is not used.

Thank you for your understanding.