Do I need a photo extension if I only want to order prints???

No, you don’t.

Order professional prints of your photos in Photos on Mac - Apple Support

Not for prints. I have them done locally thru Costo and/or Sam's Club.

However, after September 30th Apple will no longer be providing printing services for any type of print product, including prints. After that date you can order prints online by going to the website of those that offer the apps to use with Photos, i.e. Mimeo.com, prestophoto.com, Shutterfly.com, mpix.com, etc., or upgrade to Photos a \ nd order them directly from Photos.

You can also have local suppliers print them like Costco, Sam's Club, Walmart, Walgreen, etc. by going to their website and look for their photo finishing section.

I order prints from Apple consistently without any issues.