You have to make sure they sell before the promo ends.  I have also found that since eBay changed to Managed Payments, not one of my sold items had the promo linked so was charged around $150 in full fees for each item.  Frustratingly had to contact eBay help via chat everytime to get the fees refunded.

make sure you take a screenshot of the promotion in the overview page or they won’t refund your fees.  I noticed on my other account, eBay sneakily deleted all the promos I had even before they expired, none of the went into the Promo History so they refused to refund the final value fees.