Walmart has this Evenflo Big Kid Booster Seat for just $22.88 (reg. $59.97).  You can choose to pick it up in-store to avoid shipping costs.

• 2 Seats In 1 — removable back for easy transition to a no-back booster
• Easy One-hand, 6-position height adjustment to accommodate growing children longer
• 2 cup holders for drinks
• Machine-washable pad keeps seat looking clean
• Side Impact Tested: meets or exceeds all applicable Federal Safety Standards and Evenflo’s Side Impact Test Standard
• Designed and tested for structural integrity at energy levels; approximately 2X the federal crash test standard
• Energy absorbing foam liner provides added safety and comfort
• For Use as High Back Booster (with backrest)

Thanks, Hip2Save!