With this extension you can change product quantity,before you add product to cart, just by moving the slider. When you move the slider, price updates automatically. This extension requires vqmod(so all your files won't be overwritten). You can easily customize min and max default slider values, and, also, you can change style as you want)
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It perfectly works with Live Price Update extension: http://youtu.be/GdmVQZySoiM?list=UUEEF5l8PsZ7ffharP1h_xYQ
Live Price Update Extension: http://www.opencart.com/index.php?route=extension/extension/info&extension_id=12489&filter_search=live%20price%20update