USPS Informed Delivery emails have tracking pixels. But all the mail scans are just attachments to the emails. You can configure email client to not load any remote content and the they will still get rendered pretty nicely. I was pretty surprised to see those attachments in the first place, because some of the scans were quite large (a few hundreds KBs).

I can't say for sure, but haven't worried about it since I opted to _not_ load remote content in e-mail messages.

Some of the email themselves have ads too. For instance the latest one for me has a Chase banking ad.