Ok, I discovered this a little while ago and everyone had mixed reactions. I already explained it on a video, but I'll make a guide here as well. (Last way to clear any confusion)

Anyways, in the video it shows me pausing upon choosing the correct mailbox from levels 7-10. How I do this is simple enough. Select the right mailbox using the spacebar, then half a second later, press the pause button on the screen. If done correctly, you'll skip the transition from the "Great!" (or whatever it translates to depending on the language) and instantly loads the layout of that level. To save the most time, what I've done was unpause immediately after it loads and fire the laser if it showed the correct path. Unless, if it doesn't show the correct path, just wait for the mirrors to be proper.

Note: Pressing the P key will not make the glitch happen, however this happens half the time but I still wouldn't recommend it because of being slower to make the glitch work.