Postage Saving Envelope, #6 5/8, Commercial Flap, Gummed Closure, 6 x 9.5, White, 500/PackQuality Park Security Envelopes have a reflex blue pattern that protects sensitive client information because it obscures the details of the contents, even when held up to light. Single window envelopes display the recipient's address for simple bulk mailings. These postage saver flat mailers are made from a more durable 28 lb paper than standard business envelopes so they will stand up to mailings.