National postal operator seeks to provide the universal postal service. Consequently, obstacles may be created. While providing the universal postal service, the operator has the costs, which tend to be growing. In this article, we are going to calculate this trend and try to predict the next outcome. According to the costs, we should pay attention to the price of letter mail, which is connected to the costs. In this article, we are going to calculate correlation between price of the letter mail and the amount of sent letter mails. The article focuses on the importance of the postal services. Importantly, we should focus on the universal postal service, because nowadays, there is a different situation on the postal market. This article will use statistics data from Slovakia postal market, while calculating the results.