With orders of $25 of more, Amazon offers Prime members the Sealy Posturepedic Allergy Protection Pillow Protector 2-Pack, model no. 3357SLY, for $4.94 with free shipping. That's the lowest total price we could find by $3. It features a zip closure and fits pillows up to 20" x 28".
Note that this item is part of Amazon's "Add-on program"; it can only be purchased with an order of $25 or more. It also has an estimated shipping time of one to two months.