Is it too late to use promo I received for $1 selling fee?

[smileysuethc]

I forgot about the promo I received:

"$1 final value fee per item. Valid on 10 sales. "

I sold an item a few hours ago but forgot to activate my promo.  It's the difference of $14.70, is it too late?

Can I cancel the order and redo it so that I only have to pay the $1 instead of $15.70?

Thank you so much for your help.