A commercial flight is not defined by the salary of the pilot but probably rather by the commercial intention of the operator.
Common sense suggests that a commercially operated aircraft requires a commercial pilot.
From the EASA hp:
The term 'commercial operation' is defined in Article 3 of the Basic Regulation as follows:
'Commercial operation' shall mean any operation of an aircraft, in return for remuneration or other valuable consideration, which is available to the public or, when not made available to the public, which is performed under a contract between an operator and a customer, where the latter has no control over the operator.
from caa.co.uk:
The EASA PPL(H) allows you to fly EASA Helicopters registered in the EU provided that you maintain a valid medical certificate and ratings.
Licence holders can fly on an unpaid basis as a Pilot in Command (PIC) or co-pilot on non-commercial helicopter flights.