Nick666 is correct regarding Burgas airport which in fact started this checking last season, Also the correct insurance etc you must have even if you say you are not charging for the transfer. The other problem you will have is that people do not like to be transferred in RHD vehicles.
One other thing that you will have to take into account as from last year if you are thinkiing of using Burgas airport is that you will be checked that you are a legally registered company, and anyone that works for you is also legally registered. As said by the previous poster there are many illegal companys that offer transfers, this will and has to be stopped which Burgas airport is now doing. This year all paperwork will have to be produced to get your pass which as yet we still do not know how much this will be for the season.