How to print invoice or receipt?

What company's invoice?

If Apple, you click on the order number in the email:

That should bring up a log-in screen:

After signing in, there should be a small "View Invoice: link in the upper of the resulting screen:

Not easy to spot of some devices

You are most welcome!

...Why do they have to make it so difficult?

I've often wondered that myself! Only Apple knows.

By the way, those electronic invoices only stay available online for about 18 months, so it behooves you to print out the invoice—or save it as pdf to a local device—as soon as you get any new Apple product. That's what we do.

Several "Save as PDF" how-to articles are available here (link to multiple Apple search results):

Apple - Support - Search