In Sofia, taxi services are generally cheap, but you need to be cautious of scammers.
Recommend you to use one of the two popular companies:
Yellow, phone +359 91119 , site: https://www.yellow333.com/en/ , or install the app it is very easy to be used
app: https://apps.apple.com/us/app/yellow/id1042670745?ign-mpt=uo%3D8
OK Supertrance – phone: +359 973 2121, https://www.oktaxi.net/en
Always carry cash, as it’s often not possible to pay with a card.