MarketPress: when adding a product to cart, quantity is always x2

I'm a bit at a loss here...
When I'm adding a product to my cart, the quantity of the products added to the cart is always double the quantity I selected.
If I'm not mistaken this happened after the last MarketPress update.
Please note that I have copied and customized some of the functions present in marketpress-templates.php file inside my theme's functions.php.
Those customizations were from the previous version of MarketPress. However I can't find anything that would justify this behavior...
You can check it out here: http://makrismusic.magazi.org/store/products/fender-squier-bullet