Park Tickets through Undercover Tourist or Tripster

if i buy through park tickets through a third party and then need to add a day during the trip…i know i will pay for the additional day but would i also pay the difference between the third party price and Disney price? How does that work? Thanks!

No. You’ll only pay the difference between the VALUE of the two tickets, not the price paid for the tickets.

I purchased a 3-day ticket from undercover tourist and it was obviously cheaper than purchasing from Disney. We then decided to add on a day at the beginning of our trip and I was annoyed that I had already purchased the tickets and linked them to MDE and that my only option was to buy the additional ticket through Disney at their cost.
I priced out what I thought would be an additional day and what a 4-day ticket would have cost me through UT and then called the ticketing line at Disney. The CM could see that I bought the tickets through a third party and crunched the numbers and it turned out the additional day was even cheaper than if I purchased a 4-day ticket through UT. I’m not sure how that happened, especially since the new starting day was 1/1/2020.
So, somehow, they are able to calculate how an additional day would have cost you if you had purchased your tickets through a third-party and won’t charge you what Disney themselves would charge you.