B2M = Back to MAC. When you save 6 empty containers of MAC products (basically anything except wood pencils), bring back those 6 and you have your choice of any lipstick excluding ones that come in colored packaging like the viva glam lipsticks or collections like Lure where the packaging is different.
the items don't have to be empty either. if it's something that you know you're not going to use anymore, save it for b2m.
and it can be a mix of anything. like an e/s pot, a foundation bottle, a mascara tube, 2 lipsticks, and a lipgloss tube. but you only geta lipstick.