I was over in Sunny Beach in May and I got a great deal booking online through
www.carhire3000.ie ( they are part of the TravelJigsaw Group based in London ) and they used Thrifty in Bourgas Airport where I picked up the car. The car was a basic model didn't look fancy but felt sturdy and lively to drive, very good on fuel consumption. I also had an "Excess" waiver insurance through my own insurance company which helped keep the cost down.