fwiw - there certainly are some zips that cross state borders. Wikipedia says:
Because ZIP codes are intended for efficient postal delivery, there are unusual cases where a ZIP code crosses state boundaries, such as a military facility spanning multiple states or remote areas of one state most easily serviced from an adjacent state. For example ZIP code 42223 spans Christian KY and Montgomery TN, and ZIP code 97635 spans Lake OR and Modoc CA.
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