Any padded envelope should be fine.
I send mine to The Darkroom, and they give you a pre-paid mailer when they send your negatives back, so I just use that. It's basically just a plastic bag with an adhesive strip, no padding at all. Works fine, I haven't had an issue.
Film isn't so sensitive to heat that it just dies, that's mostly a concern over the span of months to years. And even the radiation from being on a plane won't hurt it. It won't get x-rayed through regular mail either, so you're good there.