Sounds like the taxis from some of the posts i am reading have a crooked meter. I used Uber in Poland and they were really handy.
For Bulgaria, I used this transfer service and pre-booked my transfers. https://vtravel.bg
They were excellent, had good reviews, and had safe drivers and a nice van.
I didn't use any taxis - personal preference as I'm just not a fan of cab rides.
I am reading mixed opinions on the legality of Uber in Bulgaria - maybe someone with recent experience there will weigh in.
The apps Yellow Taxi and Taxi Me work great. I used it for an early morning taxi to the airport and had no issues