In free or freemium games, money causes the least resentment if you can't use it to buy advantages. This "Cash Clown" seat on a team sounds a bit like an advantage - not a combat advantage, but one where money lets you buy your way onto a team otherwise composed of talented players. I don't like it.
Less inflammatory sources of revenue are Convenience - saves you the work of grinding (Existing: you can get shards by grinding Bronze, or by donating. I think donating to upgrade cards would be great too. Spend an hour at your job and donate = spend an hour grinding FGs) or Fluff - decoratives, doodads, and kudos (This is surprisingly profitable. Some people have money to burn and either love the game or are obsessive collectors of shiny things. Icons, titles, banners, different card art, and forum privileges could all work here)
There's a great video by the Extra Credits guys on properly monetizing a free game. Properly being "without making everyone rage" as well as "bringing in good cheddar".