It really shocked me when I found out that the dealerships just line it up by eye and screw directly into the bumper with self tapping screws. Everything else on this car is well thought out and upscale, but that just seems like a hack job, especially when you unscrew the plate and see what it looks like underneath.
I was hoping VW dealers used the same size screw every time and it's disappointing it's not a specific size. I did see the stainless VW screws on Amazon, but according to a response by the seller in the comments, they're M6 screws. I measured the diameter of mine last night and they're somewhere in the 4.5mm- 4.7mm range and 13mm long. 13mm is about 1/2", so it's possible the dealership used SAE screws. I'll call them on Monday.
ACE Hardware is where I went yesterday. That's where I've gone for years to buy stainless screws, but nothing matched up with what I have. In cases like this, I've had great luck with Chinese sellers on eBay, believe it or not. For some reason, they have sizes you can't find anywhere else, and it's dirt cheap.
If it turns out to be M5, I'll order these:
https://www.ebay.com/itm/M5-304-A2-...hash=item1ed8149f70:m:mnEW4ZkX1LzH-asKgmcEQGw
I really don't want to step up to a larger size until the threads strip. If that happens, I really like the idea of wellnuts. Thank you, akimmel! However, I'm not sure I like the space behind the license plate, even though it protects the paint. Doesn't it rattle?