Alright, this is really weird. I did just what you said in my custom menu and entered the full link for the Payment page, and it worked perfectly. It smooth scrolls on the single page, and you can easily get to it from the Order Form page when you navigate away.
Then, I did the exact same thing with the custom menu for the Choose Your Song page, and something strange happened. If you click Choose Your Song in the menu, instead of smoothly scrolling directly there, the page reloads at the top, as though you’ve just clicked on a brand new page. Once it loads (I have slow internet!), THEN it scrolls down to the Choose Your Song page. I did the same thing with the Contact page just to test it, and it, too reloads the page at the top before scrolling down, instead of just scrolling right to it like before.
So as of right now, I put full links on the menu for Choose Your song (weird behavior), Contact (weird behavior), and Payment (normal behavior). The other menu items just have the #page-id, and they are functioning normally.
I should add that when looking at the menu, both Home and Payment are underlined, but none of the rest of them are. Home was always underlined, but as far as I know, I never did anything different with the Payment page than I did with any other page.
Also, could this all somehow be affected by the smooth scroll plug-in I have installed?
Okay, thanks a ton. Let me know if you want a custom song for your troubles!