If you send a postcard to another Retro user by selecting them as a contact, rather than typing in a mailing address yourself, Retro uses the mailing address that contact has saved to their own account.
What if they haven't saved an address?
If your contact hasn't added a mailing address yet, they'll get a notification asking them to add one. Until they do, your postcard order will show the status "Waiting for address".
How to update your mailing address
Go to your Profile tab.
Tap the gear icon, next to the "Recaps" and "Share Profile" buttons.
Scroll down to the Account section and tap Mailing Address.
On the Edit Postcard Address screen, enter or change the address.
Tap Save.
If you're waiting on an order in this state, it's worth nudging your friend to add their address so the postcard can move forward.