The balloon will carry the invitations to Owl's relatives. Air mail.
6 seconds sound clip from The New Adventures of Winnie the Pooh - Oh, Bottle / Owl in the Family (S3E1).
You can hear this line at 00:14:11.840 in the Blu-ray version of the TV Series.
Quote context
[...]
- A family reunion for me? And to think you knew where to find my relatives all along.
- Ooh, I must get busy and prepare. This is going to be wonderful.
- This reminds me of the coming out party...
- Pooh, do you really know where Owl's relations are?
- Well, since I know they're not here, the rest should be easy.
- What exactly are we doing, Pooh?
- Sending invitations to the reunion, of course.
- The balloon will carry the invitations to Owl's relatives. Air mail.
- I'm going to help Owl get ready for their visit.
- I shall wait here for the guests to arrive.
- I suppose Owl's relations must be very shy, or perhaps I didn't use enough postage.
[...]