Replying to this thread:
I was buying the Evernote edition of the ScanSnap XI500 from the Evernote Marketplace. I first selected the identity I wanted to use and then selected my preferred credit card from the Chrome plugin (41.0.2272.104 (64-bit) on Yosemite 10.10.2). I scrolled down to confirm the order and noticed that the quantity had been changed to 15 (almost $7,500!).
The only thing in my profile with "15" in it is my birthday. Not sure why that would change a field named "quantity" in the source though. I just went back and confirmed that it was my selecting the identity that modified the quantity (and not my CC info).