Diana, it's quite a distance from Orlando airport to Port Canaveral, and $20 is not out of line. Ask your agent if you can still get transfers with the cruise line, or what else she might suggest, perhaps Mears Transportation (buses and large vans).
Re: ID - your birth certificate should be an original or a certified copy with a raised seal. A xerox copy will not suffice. Also, if you are married or have otherwise changed your name since birth, you should have a certified copy of that certificate too - most are not asked for them, but you could be, if the name on your license is not the same as that on your b.c. (If you booked through a travel agent, she/he should have informed you of this, but that info is also probably in the fine print on your cruise documents.)
Have a great cruise!