Delta rep booked flight wrong - I had to pay $410 at ticket counter... what other choice did I have?

Message #2
I don't think you understand, a Delta Customer Service agent took my
call, changed my flight to the wrong day, then when I showed up to what
was supposed to be the correct day, I had to pay $410 because of the
Customer Service error.  I really don't understand how you have protocol
that doesn't protect me the customer from a customer service agent from
messing up a simple change flight request.  

Message #1
I am not seeking anything free, just fairness. I fly weekly
with Southwest, United and ocassionally Delta. My travel agent suggested
to fly more frequently Delta. My original return flight was G3L66D for
May 4th at approx 3pm. On May 2nd I received a call from my daughter
that she made the preliminaries for major swim conference, so I called
Delta to change my flight (I could not reach my travel agent so I
decided to handle myself - please review all notes associated to this
reservation for history) to change flight from May 4th to May 3rd 8pm. I
called and spoke to a Delta representative and successfully (so I
thought) made my flight change and paid the fee. When I went to airport
to check in via kiosk the screen read I was too early for my flight,
however I was trying to check in at approx 6:15pm. I requested help from
counter representative and she informed me my flight was changed to May
4th at 8pm, obviously a clerical error from Delta representative I spoke
to on May 2nd. A supervisor was called over to help assist (Julianna L.)
me to correct error so I could fly on correct intended flight. Julianna
was professional, but seemed defensive and was not willinging to honor
the intended flight change without charging me $410.00. Which was the
flight fee difference? I am requesting a refund of the additional
charges. I am just very surprised on how Delta handled this mishap and I
am hopeful we can resolve the issue? I am open to receiving unrestricted
flight vouchers for a future flight if this makes helping me easier on
your end as my goal is to just recoup my extra expense as I can not
afford this type of error financially. If the originsl Delta agent would
have correctly changed my flight this error would not have happened. OR
if the Delta agent would have explained the significant flight price
difference, I would not have made the decision to check out of my hotel,
return my rental car and basically be at the mercy of Delta and stuck at
the ticket counter resulting in a $410.00 extra charge. Seems very
unfair and lacking in the integrity department? Please advise at your
earliest convenience if you are willing to help me resolve this issue as
again I can not afford with this type of error. Thank you for reading my
communication.