Givi Monokey series top boxes require special mounting brackets that are custom designed for particular model motorcycles or scooters. They usually bolt to the frame. They don’t simply attach to an existing rack on the bike. I have a Monokey E52 trunk on my Burgman 650.
Givi Monolock series top boxes use a less expensive mounting bracket that is not model specific. It is a universal adapter plate that attaches to an existing rack on the bike.
I think with the P150, you’d need to buy a Monolock series case. Givi does make a Monokey bracket for the Kymco Xciting series, but not for the People series (as near as I can figure out from their website – which isn’t all that easy to navigate).
I have an Italia Soft brand 28L top box on my People 250. When you shut the lid, it automatically locks. This is convenient, but I am very careful to NEVER set my keys down inside the top case (for obvious reasons). The Givi products require that the key be in the lock in order to lock the top box. More steps required (but you can’t accidentally
lock your keys inside the case).