the different price

I have booked two flight tickets on cheapoair on July 27,2013.I booked an American Airline flight  departing on Oct25,2013and returning on nov25,2013.I paid $2247 for the tickets. I will visit my wife with my daughter. I have to change my schedule today. so i used the live chat on cheapoair. I checked the filight price of the same flight number departing on Oct28,2013 and returning on Nov 26,2013. it is totally  $ 2410.90. and I said to her(or him,named Modesto M) that i wanted to buy that tickets. BUT Modesto said to me 'Please be advised, the fare available over the website is only for new booking , for changes different rules apply". AND Modesto told me that" If you choose to travel on 28 Oct you need to pay 642.00 USD per passenger".I feel it is so unfair.I said to her " if there are still some tickets ,you should sell them to me. so why you don't tell me about that when i buy the tickets". then i had to choose Oct 29, 2013 as my departing time. I had to pay the total $ 965.40 (airline penalty 245.00 USD plus fare difference 347.05 USD per passenger , she  found me so angry about the different prices,so she gave me a  218.70 USD discount ). I think it is so unfair for me. i think that a company should  sell the client the same price as what they can see online or they have the reponsibility to tell the customer or notify them online where they can see before they make a booking. otherwise it will be seen as a deceitful trick. AND I heard  from a colleague that i can get a cheaper payment if i reschedule the flight tickets on AA.  i checked the price of the same flight ticket, and it is $1304 per adult. I feel that i was fooled.Why Modesto didn't tell me that?  I booked some flight tickets before. and it was not the first time i changed my schedule. but it is the first time that i feel so bad. i will never buy the tickets on cheaponline.