Create an Apex trigger for Account that matches Shipping Address Postal Code with Billing Address Postal Code based on a custom field.

AccountAddressTrigger.apxt

trigger AccountAddressTrigger on Account (before insert, before update) {
	for(Account a : Trigger.new){
        If (a.Match_Billing_Address__c == true && a.BillingPostalCode!=Null) {
            a.ShippingPostalCode = a.BillingPostalCode;
            System.debug('ShippingPostalCode: '+a.ShippingPostalCode);
            System.debug('BillingPostalCode: '+a.BillingPostalCode);
        }   
    }
}