As far as the place you need to use a rental car, the place you need to pick it up and the place to return it later to, are concerned - these are entirely up you. Based on these needs however you have to choose a car rental company to hire a car from. I won't be explaining the simplest of cases, where you hire a car in Sofia for example, make your trip to the seaside and afterwards return the rental car back in Sofia. In this case you won't be requiring any additional services from the car rental company and thus you won't be charged with any additional fees you might worry about.
More interesting are the cases where you need to pick up a rental car from one place and return it to another. Lets explore this scenario some further. Imagine you are arriving with a flight to the Sofia airport. In Sofia you rent a car to continue your trip to the seaside for your holidays. Later however you decide to take another flight home (e.g. an earlier or more convenient flight) from the Bourgas airport. You make all the necessary arangements with the airline but what about the rental car? If you haven't thought of this possibility in advance, you might be required to pay a large ammount of money as an extra service for someone from the car rental to come pick up their own rental car. Luckily for you most car rental companies have offices in most of the major cities in Bulgaria - Sofia, Plovdiv, Varna, Bourgas, etc. That's why my tip to you is: do not take any chances and ask about return options and fees (if any) before renting a car. You might never have to use these options, but it's best to be informed and prepared, than to be cought off-guard. From my experience most of the top quality car rentals won't charge you with additional fees for returning a vehicle to another office especially if you don't make your reservation last minute.
The most complex and therefore the most expensive scenario is the one, where you pick up and/or return the rental car to a place without a car rentlal company's office present. In this case you will be charged with a fixed fee according to the service price list of the respective car rental company.