Won't the 100dpi images print fine commercially at 300dpi, if they look fine on screen. Or am I missing something in my understanding? Maybe what is printed is not the same resolution as what I see on screen?
You can export an image at any resolution but that is not going to help. 100dpi is about Windows screen resolution. It will look soft and out of focus when printed. Honest. 200dpi is pushing it. 300dpi actual image resolution is best.

I have printed a thousand or so of my stereogram images, plus I worked in advertising and graphic design for over 40 years.