Validate a UPS 1Z number before you call any tracking API - the check digit is free

If your app accepts tracking numbers from users, you are almost certainly treating two very different failures as one. "This number does not exist" and "this number cannot exist" look identical at the API layer - both come back as not found - but only one of them is worth retrying. UPS parcel numbers give you a way to tell them apart for free, before any network call. The last character of a 1Z number is a check digit computed from the fifteen before it. Disclosure up front: I work on 24hTrack, a free package tracker.