That all necessary green paper or what ever has been exchanged as necessary.
That the recipient's postal address is accurate and if they have a secure mailbox.
The recipient's preference for "insured" and/or "signature required".
(If the recipient has a secure mailbox, requiring a signature can be a hardship, fraught with it's own risks.)
I generally recommend mailing tickets via US postal service using a double envelope as follows:
Make the inner envelope by folding a sheet of paper as you would fold a letter (typically in half top to bottom, and then tri-fold side-to-side).
Put the tickets inside and tape or staple the ends of the inner envelope.
Write the address on the inner envelope, and place it in a conventional envelope, addressed in the usual way.
Be very careful to make the zip code clear enough to be accurately scanned by automatic mail sorting equipment - no smudges or crooked digits.
Mail the tickets "Certified", which gets you a tracking number.
If the recipient's mailbox is not secure, consider "insured" and/or "signature required", AT THE RECIPIENT'S PREFERENCE.
Email the tracking number (or tracking URL) to the recipient.
If the recipient wants the paper survival guide
Consider a sperate envelope (would the weight risk the tickets?)
Which sometimes arrives first
Fold once lengthwise
It probably requires 2 stamps