Code:

case $zipcode in [0-9][0-9][0-9][0-9][0-9]) ;;
    *) echo "$myline"; echo "zipcode warning: $zipcode" >&2;;
esac

If it's five digits then it can't be bigger than 99999 or less than 00000, so that check seems superfluous. Also you seem to have a logic error there (it cannot be less than one and bigger than 99999). The check for bigger than 0 still needs to be done but it complicates things a bit so I didn't want to dilute this example with the handling of that.