This simple plugin applies coupon restrictions when the backorder item is in the cart. This coupon restriction will only apply if the product is set to backorder and stock is less than 0.
Usage is easy – there is no configuration, just added two fields to the Usage restriction tab under the Coupons menu.
Two fields are Backorder Restriction and Backorder Restriction Message. You need to allow Backorder Restriction to activate this feature. You can add your custom restriction message in the Backorder Restriction Message input box, which shows on the cart page or checkout page.
Note: Coupon restriction will not apply if the cart has other instock or outofstock products and backorder products. It will only apply if all products are on backorder in the cart.