It is possible to use the Scanning Service with a P.O. Box address.
However, it is imperative to select the "P.O. Box - Standard" category at the post office.
https://www.post.ch/en/receiving-mail/receipt-locations/po-box
Conditions for Using the Scanning Service with P.O. Box Standard:
When utilising a P.O. Box Standard, you may access the Scanning Service. Please ensure that both your P.O. Box address and the corresponding residential address (or vice versa) are registered in ePost, verified, and that the Scanning Service is activated.
Separate P.O. Box and Scanning Service - Not Permitted:
If you are using a separate P.O. Box, unfortunately, the Scanning Service cannot be used. The service requires a residential address, which is mandatory for activation of the Scanning Service.