I'm not sure if it really works or if i was just lucky, but last night I bought two GP supply cases (different times) and i got an R870 SE (1st) and a Dragonub SVDS (2nd) for 7 days each.
But there was a catch to it ... i put my cursor on the top corner of the buy button. I then proceeded to click buy and left the cursor there, pressing enter to confirm and open the case.

How i placed it is shown in this horribly edited picture that took me 6minutes to make

If anyone would like to try and confirm whether it works or doesn't work then please do and take screenshots or record a video. Thanks