Linenspa via Amazon offers its Linenspa Encasement Mattress Protector in seven sizes from $19.99. Plus, Prime members receive free shipping. (Non-members can spend $35 or more to qualify for free shipping; otherwise, shipping starts at $7.58.) That's at least a buck under last week's mention and the lowest total price we could find by at least $7. It features a breathable fabric on the outside and a waterproof layer on the inside.
Of note, Linenspa Encasement Pillow Protector is also available in three sizes starting from $8.99.