I usually don’t decline requests like this, but they don’t come very often, so they’re easy to handle. I always let my sales go one day longer than I advertise, and if someone asks for another product to be on sale and I know it’s not going to be for a long time, I’ll usually just give them a coupon. It just seems easier to me. The point of a sale is not to narrow people’s choices. It’s to give an excuse to email everyone and remind them that there’s stuff to be bought.
If it actually worked, I’d just price everything at sale price all the time and not bother with promotions and all that nonsense. But people don’t think to buy stuff unless there’s a big promotion going on, so you sales are really just an excuse to remind people you exist. Once I have someone’s attention, I see no reason to not give them a discount on something if they ask for it, since that’s the whole point of the sale in the first place.