Don't save to the database if we are working with sessions, save to the session when save() is called instead.

includes/class-wc-customer.php

@@ -1012,10 +1012,16 @@ class WC_Customer extends WC_Legacy_Customer {
 	}
 
 	/**
-	 * Save data (either create or update depending on if we are working on an existing customer).
+	 * Save data. Create when creating a new user/class, update when editing
+	 * an existing user, and save session when working on a logged out guest
+	 * session.
 	 * @since 2.7.0
 	 */
 	public function save() {
+		if ( $this->_from_session && ! $this->_is_user ) {
+			$this->save_session_if_changed();
+			return;
+		}
 		if ( ! $this->_is_user ) {
 			$this->create();
 		} else {

tests/unit-tests/customer/crud.php

@@ -416,6 +416,15 @@ class CustomerCRUD extends \WC_Unit_Test_Case {
 		$session = new \WC_Customer();
 		$session->load_session();
 		$this->assertEquals( '124 South Street', $session->get_billing_address() );
+
+		$session = new \WC_Customer();
+		$session->load_session();
+		$session->set_billing_postcode( '32191' );
+		$session->save();
+
+		// should still be session ID, not a created row, since we are working with guests/sessions
+		$this->assertFalse( is_numeric( $session->get_id() ) );
+		$this->assertEquals( '32191' , $session->get_billing_postcode() );
 	}
 
 	/**