Walmart offers the Summer Infant Deluxe Folding Booster Seat, model no. 13050, for $15.29. (Amazon has it for the same with free shipping for Prime members.) Opt for in-store pickup to dodge the $5.99 shipping charge. That's $2 under our May mention and the lowest price we could find by $4. It features adjustable height and tray position, and has a 3-point restraint system.

Update: The Walmart item has increased in price; however, Amazon still offers it for $15.29 with free shipping for Prime members.