B2M = By returning six  M·A·C primary packaging containers to a M·A·C counter or M·A·C Cosmetics Online, you receive a free M·A·C Lipstick of your choice.
The MUA write "B2M" with a sharpie on the back of the lipstick,eyeshadow or lipgloss so you won't return it (the sharpie usually cover the item name).
So, we have a problem ey?
Solution: Watch the video!
- Cotton Ball