Honestly, I reckon Mythicals should be labelled level 30, so that this issue doesn't come about until level 15, which is when you get a mythical from levelling anyway.

Also, silver + gold boxes should be disabled until lvl 15 as well, so that people don't waste tokens with absolutely no chance of myths.

Yeah, Glados is right there but whether or not the player can open a box before getting to level 15 is another story.