48oz is equivalent to 3 pints, so it's nearly a half gallon (it's actually 1.5 quarts). I'm pretty sure the 1st mix-in is included, but any more are extra.
may i recommend the banana split, it's so good. i got a pretty uber deal since i work next to a coldstones (in the food court) so i get 10% off, i think mine cost around 11 dollars (including the discount).
the t shirt only comes in white, i tried asking for a black one but they said it was only for employees T_T white is not bad i guess