Depends on the part. Volvo is usually decent quality, but they love to overcharge by about 200% over the OEM retail cost if you can figure it out.
On my 31 diesel, their cost for a starter solenoid was over $475 bucks. It was just over $300 bucks to buy the solenoid ATTACHED to a new starter (still volvo)!
I bought the correct solenoid, good quality, from boatfix for $15 bucks.
I can't remember the cost difference for a drive plate on my 31, but it was again about double and I spent the time to research to true OEM for the exact same part (Torflex).
For little things like oil and filters, usually the difference is not so great. Use whatever will make you maintain it at the proper intervals. If being able to find it easier and use not mfg parts will cause you to not miss scheduled maint, then do it. If you can get Volvo easily and not much more cost, use that.
They're a bunch of crooks on a lot of things. Do your homework.