The purpose of this thesis is to study which of two different sales services cost more to use. This thesis works as a guide for any entrepreneur that is thinking of starting an online business. With this study, we are able to get a current picture of online sales cost and how they compose. The goal of this study is to find out the differences between two different online sales platforms. This study is needed in order to achieve that. The framework of this thesis consists of books and digital publications about ecommerce and Amazon.com. Ecommerce, Amazon.com and fulfillment by Amazon are presented in the theory part. Thesis is quantitative, which presents cost structure of two compared sales services. That helps us to conduct the calculations used in the comparison. The study is made by calculating different costs by using two different products with different sales quantities. The conclusion of which one of the compared sales services costs more in different situations is at the end of the thesis. The conclusion was made by exploiting both research method and theory. Thesis gives comprehensive foundation about cost structure and clearly shows the cost differences between two sales services, that is why the thesis gives needful information to entrepreneurs for choosing their own sales service.