one device, US$10 per 30 days from activation.
Coupon once bought expires within 30 days (and a further 30 days after activation), so you can only renew your coupon every month.
https://www.usmobile.com/wifi
If you have two devices, that still leaves you with US$20 per month, which is a steal!
Worked seamlessly on DL international, JAL international. I will next test it on Singapore Airlines two weeks from now.