Yes! If you have an item in your mailbox and you are waiting on another item to arrive to ship them together, simply wait for that second item to be uploaded to your account so you can ship your items together.
Note: Mail items will be shredded in your account after 30 days unless you place them on hold so be sure to mark an item to be placed on Hold to prevent it from being shredded. You can also turn auto hold on in your account preferences.