There is some special power to the word FREE. People just love free stuff. You can get a 99% discount on a product, you can sell it for one cent, but it will never get as much attention as a product that is free. What’s also interesting is that free-plus-shipping also works very well, despite the fact that the buyer must use his credit card.
The second important fact to know is that once people pay for something, they are much more willing to buy from you again. So, if they pay for the shipping of a free product, next time they are much more likely to buy your high-priced product. You can double or even triple your revenue by adding free-plus-shipping products - it’s so powerful.
You can offer other free products, like free memberships or free ebooks, but they will never be as powerful as a free product-plus-shipping.