Giving your zip code to a cashier could lead to you getting more junk mail and marketing calls. 

A consumer watchdog says most shoppers believe the zip is needed to complete credit card transactions but it is irrelevant when making in-store purchases.

They say stores use the zip to combine with addresses, phone numbers so other specific details can be traced. This information is then added to a database or shared with third parties.

They use that info to send junk mail.... and make marketing calls.

YIKES!  Read more here

-AJ