If you look at the “offering rates”, you can see the chances of this. This is a “Rare Pack” which happens 0.05% of the time (or 1 in 2000).

The chance of a Rare Pack has stayed the same, but some of the newer packs have alternate types of “Rare Pack”. This set has the “Regular pack +1 card” (that extra card is a shiny) which has a rate of a little over 1 in 20, and this set is the first set to have a “Themed Rare Pack”, which in this case is all of the shiny megas and has a rate of 1 in 20000.

All of this info is in the “offering rates” at the bottom left of the pack screen.

it’s a god pack, you were very lucky, im 6000 cards and no god pack lol