I bought the add on plugin mentioned above which works fine but imo needs a few improvements such as the ability to for global addons to be done by product id vs just by category. If you don’t do global you have to copy each one over again and add it(which is just a cut and paste). Not every product is the same so this would be a good improvement. The spacing is also out of wack with my theme the retailer, it pushes the add on drop down or text area(what every addon you choose) further down the page. This could just be a css issue but i have been to busy adding products to mess with it. Otherwise it is a solid. Hope that helps.