Like Ducimus, I have to question the "strategies" people seem to be using. I get no option to send Velvet or Spice by Caravan, ever. Even if it's the most prominent good in the city, I get the option to send a Beef caravan when there's only one Beef, but can't send Spice when there's 11 Spice.
I was making more money by buying and selling Velvet and Spice myself, looks like I just wasted 45k on a Trade Permit. Man this game is frustrating.