In the flush-mounted version, in my opinion, an additional component is needed, which is usually sold for a good 100 euros extra. It is called, for example, "iBox universal basic body" and can be found online for half the price.
Morning
For the installer, there are several components included in the additional price but usually not listed.
For a surface-mounted shower fitting, usually needed are: at the end of the pipes two fittings/wall flanges and the fitting itself, to grasp and assemble two threaded connections.
For a flush-mounted shower fitting: 4 fittings for the pipes, flush-mounted installation body, some pipe, and a wall connection elbow. Here, in two operations, 5 threaded connections have to be grasped and assembled.
The costs
Flush-mounted installation body 65€, two fittings 20 €, wall connection elbow 25€, difference of the fittings 46€ so the installer still has 44€ left for the extra effort of assembly and for risk and profit.
Olli