Since i have moved basses around Europe for fifteen years here's my (barely) $0.01.
The first rule is to have a robustly made box to transfer the bass. In the box you will put the instrument with the strings detuned, the bridge off the strings and the soundpost down. The bass has to be well stabilised in the box, with rolled carton reinforcements placed properly and with foam bubbles in the sensitive places (mainly the neck).
It is understood that the box must be easily inspected by any authorities in border crossing, so the closing of the box has to be safe but easily opened and closed.
If you plan to travel with the bass the best solution is any bus service which operates around Europe (is FlixBus).
The instrument will be placed in the baggage compartment and you will take it safely out in Spain.
From the other hand if you plan to send the bass alone please choose any decent cargo company which operates between the two countries. Make clear to them that the are going to carry an extra fragile instrument and buy an insurance higher than the instrument's cost giving them the insurance's details.
They are going to be extra careful.
Following these simple rules you'll have your bass safe in your country.
If you opt for the bus service the cost of transporting your bass is very low, since it is considered a bulk baggage of the passenger.
If you choose the cargo service the cost is going to be less than 200 euros. As i remember transporting a bass from the Czech republic to Greece i paid 150 euros.
Best wishes from Greece
Mike