@RedOct @ojensen There isn't a good explanation but I doubt they track only UPS links (nor for the purpose of getting your tracking ID) you visited but rather for every single website with their trackers, so it's sort of like they have access to your browser history in full detail (which is very convenient for targeted advertising).
There is a non-negligible chance they don't even parse this link to wait for your mail together, it's a useless data by itself.