You're going to find that a large number of resorts charge outside vendor fees, and they can get quite expensive depending on what you want and where you actually end up. Some fees are as high as $800 or $1,000. A lot of brides just pay the fees if there is something really particular that they want, like an outside photographer or videographer. Even if you decide on an outside vendor for flowers, you will need to check the resort policy regarding items like that.
This site will give you loads of information depending on which resort you decide on. Definitely check out the resort reviews and see what past brides have said about the resorts you're considering. Then search the forums and read what is being said by current brides about the specific resorts.
And of course check with your travel agent! That is one of the best possible sources of information you'll get!
Good luck and happy planning!