If a buy a megaticket, do I have a chance at winning a random potion or just the potion I selected?

A megaticket gives you a chance at any potion the lottery gives out, not just the one you selected. c:

Yay! There are several potions I'm interested in right now. ;)

Iirc the potion lottery draws a random potion, then a winner out of those that bought tickets for that specific potion. The mega ticket gives you one entry towards each potion.