13 Deals offers the Geared Up Front/Back Seat Rapid Charger for $12.49 with free shipping. That's $16 off list and the lowest price we could find, excluding the mention below. It features two dual-port USB chargers connected with a 6-foot cable. Deal ends October 31.
Don't mind waiting? Wayee via eBay has it for for $8.10 with free shipping from China. Expect delivery in two to six weeks.
These retailers offer some of the most consumer-friendly price-match policies anywhere.