there are different transfers companies you can book, and most you can book from the internet. One of the cheapest you can book online is showing a price of 54.36 EUR for 2 people, but I am not sure about the conditions at shuttledirect.com, if you have to be waiting until the minibus gets full of people, or if it works as a taxi.
If you are flying between Bratislava and Girona, then Ryanair has flights almost everyday. The price is about 150 euros one way, or it could go down to 50 euros if you book in advance, meaning 2 months in advance.
Be aware with Ryanair abusses with luggage.