Home Depot offers the Commercial Electric 100-watt Power Inverter, model no. 7821C, for $5. Choose in-store pickup to avoid the $5.99 shipping charge. That's $6 under the lowest total price we could find for a similar inverter elsewhere and the best price we've seen for any such 100W inverter. It features 100W AC output, USB, swivel cigarette plug head, and indication LED.
June brings some big deals and sales, but you're better off postponing certain purchases.