Those "free trial pair" certificates are just a marketing ploy to get you in the office
all new contact lens wearers are fitted with a trial pair. the doctor will discuss with you your eye conditions, habits, hobbies, etc. and help you pick the best lens for you and your lifestyle.
if you have never worn lenses before, the office staff will train you on insertion and removal, as well as cleaning procedures, and the doc should have you come back in a week r so with your lenses ON so that he can see on a microscopic level how your eyes are doing
if you live in the US, you should not be able to purchase lenses without a prescription, a contact lens or glasses Rx is JUST THE SAME AS a prescription you fill at the pharmacy.
EDIT: if the first lenses don't work, or you just aren't sure, ask the doc to try something else. he might not want to go through four different lenses with you (with four follow ups) but he should work with you until you have maximum comfort and visual acuity