Sears offers the Battery Tender Dual USB Port Car Charger, model no. 021-0161, for $4.99. Opt for in-store pickup to avoid the $5.99 shipping charge. That's $8 below the lowest shipped price we could find. (Other retail stores charge even more.) It features one 2.1-amp USB port and one 1-amp USB port.
Of note, Shop Your Way Rewards members will earn a Sears credit with this purchase and may be eligible for additional credits and discounts.