If you have multiple location, then, yes you need to click Update button to immediately perform an item synchronization using the current settings. Note this will import all items from the server Inventoria, but it will only update the quantities from items at the selected location.
Items will not show in Express Invoice, when using Inventoria as the server program, you can update Inventoria's inventory levels from other programs, that's all, it wll not sync any other data.