This help page explains our discount settings in detail:
As you can see, you can create discounts good for All Cart Items, which take either a percentage or a single flat amount off the buyer's item total or shipping charges, or discounts good for a specific product, which markdown the per-unit price of that product by a flat amount.
If you'd set up a coupon for All Cart Items that you would prefer to have applying to just one product, you can go into Seller Admin > Cart Discounts, select the discount and click Load, then change the Product menu from All Cart Items to the product that coupon should apply to, make sure the Discount amount is correct as a per-unit price markdown for that product, and click Submit to save changes.
Take care that you DO NOT change the Discount Name, so any changed settings would be applied to that existing discount. We use the Discount Name to distinguish one discount from another, so changing the Discount Name would actually keep the original discount and create a new discount under the new Name with the new settings.