USPS tracking numbers are at least 22 digits long (depending on shipping method; some can be 27); I don’t think ease of data entry was a priority for them.
22 digits and they somehow still manage to reuse tracking numbers. 10^22 (or 10^20, if we assume some encoding overhead) valid numbers should be able to uniquely identify a lot of mail.
For the ones you mention that have 22 digits, the first 8 digits (if memory serves) identify the class of mail service and other metadata. So that leaves you with 10^14 at most, assuming there’s no encoding overhead / check digits.
> USPS tracking numbers are at least 22 digits long
Actually, that’s wrong. There are shorter ones.
Examples:
82 000 000 00 - Global Express Guaranteed
EC 000 000 000 US - Priority Mail Express International
CP 000 000 000 US - Priority Mail International
There’s also foreign shipped packages that will follow various standards that aren’t 22 digits, yet are still technically a valid USPS tracking number.
22 digits and they somehow still manage to reuse tracking numbers. 10^22 (or 10^20, if we assume some encoding overhead) valid numbers should be able to uniquely identify a lot of mail.