Valve broke MvM:
https://www.reddit.com/r/tf2/comments/3iott9/mvmpress_f_away_from_upgrade_shop_after_buying/
steps:
go to Upgrade Station
get some upgrades
with the menu still open, back away until it disappears
hit F
hit Refund
hit F
hit Cancel
repeat
if you want to get easy loot or the Gear Grinder badge, now's the time to do it
Note: any illegitimate upgrades will be lost if your team loses a wave.
update: I just got the Gear Grinder badge, ironically without doing the glitch since the other players wanted to play legit. now the only badge I have yet to get is the Mecha Engie badge