Drugstore.com cuts $3 off select Robitussin items. (Discount is applied in-cart.) Shipping starts at $5.99, or get free shipping on orders of $25 or more. With prices starting at $2.59 after coupon, that's the first sale we've seen on this medicine at drugstore.com. Note that the best deals are found when you spend $25 or more. Deal ends November 24.
A best bet is the Robitussin Peak Cold Nighttime Nasal Relief 20-Pack (pictured) for $5.59 before the discount, $2.59 after. (Click the coupon checkbox on the product page to get the discount.) With $5.99 for shipping, that's the lowest total price we could find for this quantity by $3.