Do anyone know what type of stock and finish do these mailchimp decks have??
Choices over at USPC are pretty limited these days. Process of elimination, anyone?
Stock: Bicycle or Bee Casino. They come in a range of thicknesses, but you don't get to pick the precise thickness, just the range. Better still, they overlap! A thick Bicycle deck can be thicker than a thin Bee Casino deck. The Bee Casino is the thicker range overall, naturally. They're probably Bicycle - Bee costs a little more and takes longer because they have to outsource the boxes (USPC doesn't make boxes large enough to accommodate the thicker deck). T11 might use Bee for their own decks, but for a deck made for a third party (MailChimp), they probably used the less-expensive option. I actually can't think of any occasion where they used Bee Casino on a third-party deck, but then again, I don't own all of them, either!
Finish: Smooth or Embossed. Few people use smooth finishes these days because of the added glide an embossed card provides. The pockets on the card's surface, as I'm sure you know, act like the dimples on a golf ball.
Coating: Standard or "Magic Finish." Magic Finish is the default for all custom playing cards - standard is only used for their mass-produced decks or by specific request.
Process of elimination: Bicycle stock, embossed, with Magic Finish coating.