One website recommended, "To avoid delay, make sure you put the coupons in a #10 envelope and then put the envelope in a Priority Mail envelope with tracking, certified mail and insurance services. I'm sure you'll just get back sending it the plain ole way but with reports of lost mail, you can never be too safe. An extra $15 spent is nothing to make sure you get your R/T flight which can easy exceed $1200 in value."

Has anyone sent in a priority mail envelope? I assume this is acceptable.