Hmm, maybe I'm missing something.
When I do as you describe, clicking on the 'buy now' button opens a new tab straight into paypal (thats the payment services I'm using) with no option to edit the quantity, although it does correctly show the number of items selected before pressing the 'buy now' button.
Just to clarify, I'm trying to get the popup cart that appears when using the 'add to cart' button to accept
1: the customer selecting a quantity before pressing the button.
2: the popup cart to show the quantity selected.
3: still give the option to adjust quantities.