To give users access to a broader variety of print services, macOS High Sierra introduced Photos Project Extensions which integrate directly with the Photos app. In macOS Mojave, Photos will transition entirely to Photos Project Extensions. Users will be able to print and update their existing projects using these new services, and will gain access to a broader variety of options.
For final purchases using the existing built-in service, place your orders by September 30, 2018.
You can begin using Project Extensions now: learn how to install Project Extensions, convert your existing projects, and create new projects.