I'm not going to describe every detail, just some of the stuff, that is not unequivocal...
If you use Dimir Charm on yourselfe (the mill ability), or you use the draw-discard ability of Izzet Charm, or Desolate Lighthouse, it's fitting with Snapcaster...
Dimir Charm is fine with Miracles, so in case I want to sideboard Bonfire, I have a 3rd there for addition, but not only for that reason... ...it's also good against Jund, Rakdos & other Grixis decks, that play Rakdos's Return (expecially against Jund) & it's also good against aggro with small creatures...
Rakdos Charm is good against grave decks, but I have other stuff against that, so I don't need much of it... ...though, it's good against U/W Flash, because it could destroy Pike... ...it's also fine against aggro, when your opponent has a board full of creatures... ...but I only need 2, thanks to the 4 Snapcaster mainboard...
Ultimate Price is for a possible surprise Angel of Serenity, when I have no counters, but I'm thinking about using more counters in the sideboard instead, because it won't resolve than, still, I'm afraid of Cavern of Souls... ...and also, I can use Ultimate Price for other options, that I can't use counters for... ...besides, I'm not going to use Essence Scatter & Negate won't do here... ...may be Dissipate, or more Counterflux would do, but they cost 3 mana & it's sometimes too much for what I'm up to... ...so Price stayes.......