Thanks everyone for your advice..
You are right, it is NOT a big amount ($100.00), but given that I made this mistake while booking some one else's ticket with their credit card was worse than the $100.00 itself..
I thought that there might be a 24 hours clause similar to that for Air Canada, but I guess not...
I will still give it one more try and call the L.A. office on Monday and hopefully, get it resoved or , as you all suggested, buy another ticket.