hmm - if you try this on our demo the modal closes and you go to cart..
but I see your using a 3rd party component com_onepage ..
as this over-rides many of our (especially cart) templates I'm not surprised it doesn't work correctly..
I can only suggest you go thru both sets of cart templates and try to see what is different...
I assume it will be some JS somewhere to close the model on a click...