There are a couple of different ways to load a document into the LetterStream website for future use.
To save your document for later use, log into your LetterStream account, hover over the My Account account tab in the top navigation bar, then click Remembered Items. The My Account - Remembered Items screen allows the user to load new artwork for documents, logos, and envelopes for later use as well as modify/delete existing "Remembered Items".
To add the remembered item to your mailing, simply look for the "Use Saved Insert" option on the Job Inserts/Review screen while building your mailing. After you have a saved your document in Remembered Items, it will be available for any future mailing without the need to reload it.