Why does GlassesUSA require an ADD number for simple sunglass readers?

I just want simple sunglasses with no prescription (plano) on top and 1.50 readers on the bottom. GlassesUSA won't let me order online without my ADD, whatever that is. I can't just tell them I want 1.5 magnification. How can I get around this without spending $80 at an optometrist? (I know, don't shop there - but I've got a $150 store credit, so I'm stuck with GlassesUSA).