Shopping Cart Button not carrying quantity into shopping cart

[tga-cupholders]

Have used similar buttons for many years without incident and have 15 buttons in use all built in the same manner. I have a peculiar issue where the quantity drop down on some of the newer buttons does not get reflected when you add to cart. The selected quantity appears as a note under the 'more' link in the cart, but the quantity remains as '1' no matter what the user selects in the button code. You can see the issue if you visit https://buycupholders.com and select multiple quantity of the first item (PlatinumPlus). If you try with one of the items at the bottom of the page, the quantity dropdown works as expected. So far as I can see, all these buttons are setup the same (most are made by duplicating previous buttons). Anyone have any ideas? 

[tga-cupholders]

Wait... just realized what was going on...

<input type="hidden" name="on0" value="Quantity"><select name="os0">

needs to be
<input type="hidden" name="quantity" value="Quantity"> <select name="quantity">

Not sure how I overlooked that