I'm sending a large yellow envelope to Florida. It contains a CD and a pack of cookies, so its about a pound. How many stamps would that be. Or do I have to pay for a special stamp at the post office? Also, should I write "Fragile" myself, because it has a CD?

The Post Office charges by weight AND by size. You can find how much it would cost at usps.com. But, you will still need to go to the Post Office to mail this as it is over 13 oz and would have stamps.

shipping goes by weight google us post office for measurement they will give yo an estimate

Postage is based on WEIGHT not physical size. Assuming the cookies weight enough for the total weight to be at least 13 oz (slightly over 3/4 lbs) rules put in place in response to the 2001 anthrax scare require the package to be HAND DELIVERED to the counter at the post office if you use stamps. While you are their, they can weigh the package for you.

bring to the post office they will tell you.