I need to know if there is a way to create this in Magento: First of all, all my products price are fixed: 9.99$.
So i want to do this: if the client bought three products, the third is free, so it pays 2 products. He can do this as many times as they want, so if you buy 6 products, there will be 2 product free, if he buys 9 products, 3 are free
Magneto supports buy x get y free promotions. Information about setting them up is available at http://www.magentocommerce.com/knowledge-base/entry/how-to-setup-buy-x-get-y-free