Why is zip code called ZIP code?

Related questions

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5. Do zip codes change often?
6. Is ZIP code one word or two?
7. Do ZIP codes always have 5 digits?
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Tags

1. #united
2. #postal
3. #efficiently
4. #acronym
5. #properly
6. #improvement
7. #letters
8. #system
9. #chosen
10. #states
11. #travels
12. #senders
13. #quickly
14. #codes
15. #written
16. #capital
17. #suggest
18. #service