With orders of $25 or more, Amazon offers Prime members the 3M TEKK Protection Folding Earmuff in Blue, model no. 90559-80025T, for $4.12 $4 with free shipping. (Non-members can spend $49 or more to qualify for free shipping.) That's the lowest total price we could find by $5. They're adjustable and purport to protect ears against noises at or exceeding 85 decibels.

Note that this item is part of Amazon's "Add-on program" and can only be purchased with an order of $25 or more.