That Nexus 6 deal from earlier only lasted a few seconds, but it looks like there's slightly less demand for the HTC One M9. The unlocked version of this device in gold is on sale for $544.99, which is over $100 off the regular price. It's a lightning deal, so hurry up.

2015-07-15 13_01_46-Amazon.com_ HTC One M9 - Factory Unlocked - Gold_ Electronics

This is the unlocked M9 and should work on any GSM/LTE network like AT&T or T-Mobile. As with other Prime Day deals, you need to be a Prime subscriber to get the sale price. If you're not already, there's a free trial of Prime. It's hard to go back to non-Prime, though. That's exactly what Amazon is counting on, I'm sure.

The percent of stock claimed is jumping around a lot right now, but plenty of slots are apparently still left. The M9 might not be the best phone right now, but this is a good price on a flagship phone.