Sears offers the Battery Tender Dual USB Port Car Charger, model no. 021-0161, for $10.99 with free shipping. (Weaver Equipment Co LLC. via Amazon offers it for the same.) That's the lowest total price we could find by at least $2, although we saw it for $6 less via pickup last week. 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.