@lerk No idea, but you are not the only one who told me that. From what I gathered from my experience and what other people said, purchases from Amazon, Steam, Threadless, Etsy, Barnes & Noble, and Walgreens may show up in there. PayPal receipts apparently don't.
I don't have any extra information about that. I wonder if there are settings that actually prevent the purchase page from being populated – if so, they are not clearly marked.