Monoprice sitewide discounts are nothing new, but $50 off a $150 order of Monoprice-branded products (with code 50MASTER) is basically unheard of. The only catch is that you’ll have to pay with MasterPass, which is free to sign up for, and works with non-MasterCard credit cards, so it’s really not much of a catch at all.
This landing page has a few ideas for products, including monitors, audio gear, and even PlayStation VR (Update: Sold out), but the code isn’t limited to those products; it seems to work on everything Monoprice-branded.