If it wasn't applied at checkout and you paid, it cannot be applied retrospectively.
Promotions have T & C's and ' sale by dates' to be used. They cannot be used on all items sold.
Did you before paying at eBay checkout, add the code for the promo ? It's usually added at the bottom on the left of checkout and has to be done before paying.
If not, or it didn't accept the code, then the promo wasn't applicable to the item you purchased.
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