Doh! Thanks, @awesomerobot and @Lilly! I thought I’d tried fab-paypal, but I suspect now that I did it only in the {{d-icon "fab-paypal"}} and not the register_svg_icon "fab-paypal" if respond_to?(:register_svg_icon).
Computers are so picky.
I appreciate your time. I spent well over half an hour on this on several different occasions.