We ALWAYS book with National and have been very happy with their service. The reservation you have printed is your rate and car class. If they don't have your car class available, they will upgrade you at no charge. We were upgraded from a full size to a convertible last April.
The only thing you want to be very careful with is the insurance pitch. They may (some employees do, others don't) try to get you to add insurance. If you don't want any insurance make sure they know that. I got caught by replying "yes" when the man said, "so you just want the basic coverage then?" Well basic coverage is not included in the rental price and added $100 to my overall rental price.
You will have to initial different parts of your agreement. Take the time to read what you are signing. Sometimes they try to add pre-paid gasoline on as well. Everyone finally lands in Orlando, vacation is about to begin, your excited, tired from traveling, and want to get to your hotel. Don't rush through your signing process! Take the time to know exactly what you are paying for.
Don't be nervous or worried, I have always loved National. In fact we booked with
Budget for this April (much lower rates) and I am very nervous we are not with National!