Let's try this

nixos/queen/configuration.nix

@@ -103,7 +103,7 @@
     noto-fonts-emoji
     oh-my-zsh
     onlyoffice-documentserver
-    postgresPackage
+    ${postgresPackage}
     python3
     rsync
     rabbitmq-server

nixos/queen/postgres.nix

@@ -4,10 +4,12 @@
   ...
 }: {
   #Define postgres here so this is the only place to update its version
-  postgresPackage = pkgs.postgresql_15;
+  postgresPackage = pkgs.postgresql_15.withPackages (pp: [
+    # pp.plv8
+  ]);
   services.postgresql = {
     # https://nixos.org/manual/nixos/stable/#module-postgresql
-    package = postgresPackage;
+    package = ${postgresPackage};
     enable = true;
 
     # Ensure the database, user, and ownership is set