You should e-mail them via the Airbnb platform; it is always best to keep communication here in case of any issues down the line as Airbnb only accept communication on their platform as evidence.
You can get their phone number from the Transaction - HOST> TRANSACTION HISTORY > choose from COMPLETED OR FUTURE TRANSACTIONS > click on the reservation code highlighted in red for the particular guest, contact detail can be found here
Sure you can ask for a mailing fee or ask the guest to send a prepaid self-addressed envelope and once you/ your cleaner receive it you can post back the item.
All the best,
Ama