According to the regulations, you'd have to pay 10 times the price of the ticket on the spot. If you refuse, or don't have that much cash they'd have to give you a "statement for established administrative offence". They should also take you off the vehicle.

See the official source:
http://www.sumc.bg/en/transport/tarifa-i-prevozni-dokumenti/47/regulation

In reality, you'll bribe them with 2-5 lv and you'd get away, unless you really piss them off.