Personally, I jumped right and and bought the mp starter pack. Two issues: 1) When given the mp starter pack soon after, I felt a bit silly for wasting my pizza. Suggest giving the pizza back instead, like you do for other gifts the user has already bought. 2) When Melvin introduces me to mp soon after, his comments and graphic don't quite sync with the fact that I won't be playing the AI and cannot remove his face from the screen. Might need some kind of check on whether the user has already played multiplayer.
I jumped straight into MP as well. I didn't mind paying pizza for something I got for free later on so much - I wanted it immediately, not down the road. But it was irritating that it unlocked the same deck I had already bought. I didn't really need 2 sets of the same mediocre starting cards.
