Did some searching and it seems like the cards you can get from "Pack 1" are available in the post-game normally. That leads me to believe the subsequent packs might be entirely new cards. The + packs contain the same type of card as the non + packs, but they come already upgraded. Similarly for the ++ packs.
This is what I remember from playing the game last year:
You can obtain seal stones in normal gameplay but they are somewhat rare, you can also earn them from online battles. They're probably not worth buying because you still need enough points to upgrade cards, and the game isn't so hard that you need a bunch of top level cards.
Don't buy the rub points, you earn plenty through normal gameplay.
The in-battle items (like the one that says it moves your units forward 1 square) are just a money grab, the game is not difficult.
There are some interesting items like the one that lets you upgrade a card without having to do the rubbing minigame, but at 50 cents a pop, it's not worth it. Unless you really hate the rubbing minigame. Even then, you'd need a lot of those items over the course of the game.