Envelopes are an important supply to have at hand in the office mailroom. From standard business envelopes to interdepartmental envelopes, your office needs might call for a variety of mailing materials. Envelopes are available in both single and double windowed forms. They can also be found in different sizes, including invitation size. Business documents may require security envelopes for confidentiality. Self-stick envelopes eliminate the need to moisten before sealing the envelope. Find all of these envelope types and more online at Office Depot.