LocalSend - package request
LocalSend has mobile versions of its program, so it seems perfect for postmarketOS.
LocalSend is a cross-platform app that enables secure communication between devices using a REST API and HTTPS encryption. Unlike other messaging apps that rely on external servers, LocalSend doesn't require an internet connection or third-party servers, making it a fast and reliable solution for local communication. https://localsend.org/
For your information: XMPP already has this functionality for over a decade, though some clients don't support it, yet.