PostBox is a Spigot/Folia plugin, which gives you the possibility to send items to other players.
Head over to the latest release.
Download the jar file (currently postbox-1.2.8-shaded.jar).
Drop the downloaded jar file into your Spigot plugins folder.
Stop your Spigot server and start it again.
If you want to check the authenticity of the downloaded file you can do that by verifying the GPG signature.
Ubuntu
$ curl -sSL "https://www.joestr.at/assets/software_joestr.at_0x3866986c_public.asc" | gpg --import - # Import the key used to sign the software
gpg: directory '/root/.gnupg' created
gpg: keybox '/root/.gnupg/pubring.kbx' created
gpg: /root/.gnupg/trustdb.gpg: trustdb created
gpg: key 949F21403866986C: public key "joestr.at (Software signing) <email@joestr.at>" imported
gpg: Total number processed: 1
gpg: imported: 1
$ curl -sSL "https://github.com/joestrhq/PostBox/releases/download/v1.2.8/postbox-1.2.8-shaded.jar.asc" # Download the signature file
$ gpg --verify postbox-1.2.8-shaded.jar.asc postbox-1.2.8-shaded.jar # Verfiy signature
gpg: Signature made Thu 02 Nov 2023 11:28:33 PM CET
gpg: using RSA key 0B3BC83B18C12C83948D5CF5949F21403866986C
gpg: Good signature from "joestr.at (Software signing) <email@joestr.at>" [unknown]
gpg: WARNING: This key is not certified with a trusted signature!
gpg: There is no indication that the signature belongs to the owner.
Primary key fingerprint: 0B3B C83B 18C1 2C83 948D 5CF5 949F 2140 3866 986C
To build the project you need at least a Java Development Kit (JDK) in version 17 and Maven 3 installed.
At first get a copy of the source code. Preferrably via git clone https://github.com/joestrhq/PostBox.git.
Initiate a build with mvn -Dgpg.skip=true clean package.
The compiled plugin (postbox-X.X.X-shaded.jar) will be available in the target folder.