But I just thought of a great solution, involving REDretarget that you can use for Teespring - as well as any other campaign where you do not have access to put your own tracking code on the landing pages:
Simply stick the Key Page View pixel code inside your REDretarget link! And, Boom! Now you, too can optimize your FB campaigns in oCPM - FB Pixel mode with a pixel that gets a lot more fires than the checkout pixel.