This is the unlocked international variant of the phone, but it should still work on U.S.GSM carriers like AT&T and T-Mobile. This deal at B&H Photo (via PocketNow) is scheduled to expire at midnight tonight Eastern time, so the clock is definitely ticking on this offer.
Just a reminder: The Lumia 550 launched in late 2015 and has a 4.7-inch display, a quad-core Snapdragon 210 processor, 1GB of RAM and 8GB of internal storage, which is expandable with a microSD card. Its rear camera has 5 megapixels while its front-facing camera is 2MP. It also has a 2100mAh battery.