If major spareparts, better just ask the mechanic to order and let him profit a bit. A lot of bad quality spareparts out there. Even look clean with good packaging, doesn't mean it wouldn't fail after 3 days of installation. If you brought your own spareparts, your mechanic will charge you a removal and installation fee again and you'll have to bring the sparepart to get it replaced yourself and doesn't guarantee no issues after installation. Damn mahfan!
This is from personal experience replacing the idling valve for my kenari. Total 4 rounds to the spareparts shop.

F&B
- Bring own alcohol beverage = corkage charges based on quantity.
- Some makes you sign a waiver of responsibility if you bring your own ingredients

This is from manufacturing/packaging.
Last round my mechanic showed me an original drive shaft gotten from a reputable supplier by the customer.
Packaging very good, looks legit. But few days later, customer come back said got problem, mechanic told him either the driveshaft is a lemon, or supplier stock got mixed with copy ori stuff.
Removed it and ask him to bring back to settle himself. Supplier die die insists its ori, till the customer went back to the service centre to verify and the guy there said lately a lot of copy ori which cannot be identified just by looking.
They also kena few cases like this but just consider it as lemon units and replaced it for free, as SC can claim from manufacturer directly.

So I rather let my mechanic earn a bit so I no need to headache with such hassles.

Yeah.. Bottomline is, who buy, who responsible.
So if the mechanic buys it, anything wrong, he needs to solve it, not me. I even demand my mechanic to give me a warranty period for the spare parts he get for me.
Once I've paid the 1st time, I'm not gonna pay labour charge 2nd time for the same spare part installation.

Last time lower quality spare parts are 'From Taiwan', can at least use and last a year or two.
Now, few hours after installation already kantoi.. So really not feasible to buy own spare parts to save a few bucks.
Penny wise, pound foolish.