Delta Airlines Multiple problems with "customer service" - latest is they changed how much a fare switch would be. Internet, Nationwide

I have repeated problems with Delta Airlines, causing much loss of time, energy, and now, money.   

I am currently abroad.   It was an involved flight, flying first to Chicago to pick up my 82 year old mother, then come with her on another airline to Cancun.   Now I am trying to get back earlier than expected.   To do so means I have to change flights on two airlines.

I called Delta first, who told me it would cost $150 to switch. I said I would check with my other airline, and call them right back.    I then called the second airline, who told me it would cost $80 to switch.   Based on the information Delta gave me, I said go ahead.   So I switched the first leg of my flight.  This took a few minutes.   Then I called Delta back.   But now they said it would cost $150, plus the cost of a fare change.   The total would then be over $500!

I asked for a supervisor who offered a lame apology, and said there was nothing she could do.   Now I am stuck somewhere in between my original plans, and my new ones.   I would have not switched any plans at the price Delta has said.   And now I am unsure what to do - pay their increased price - and seek to get it back.

As I said, this is not the first time I have had problems with them.   I think their call center is abroad, and their personnel are not well trained.   It sounds to me like their behavior is not only bad business, but that I have a legal issue.  In any case, this is the worst airline I have dealt with in the U.S.   Who needs all these problems?