Tires are almost never indicated on the window sticker, but they should be on the build sheet. Code R01. Either way, I believe high performance tires are standard on the GLA35 so they wouldn't show up because it's not a change. I hate agreeing with dealerships but it sounds like they're not technically in the wrong. It would have been a good thing to point out for sure, but you didn't do your due diligence unfortunately. When I bought my Fiat Spider, I knew it came on summers so I went on tirerack, found a decent set of wheels and winter tires, got them (iirc it wasn't much over $1,000 but then again that car has much smaller wheels and tires, I believe the standard wheels are 17s and my winter setup is 16s with an inch extra sidewall), and I keep them on thw wheels separate and just swap twice a year.