Usability has become in an essential aspect for the success of E-Commerce applications, especially in the current context in which there is high competitiveness because of a large number of websites that are available on the Internet. However, there is little evidence in the literature about the existence of methods that allow specialists to measure the level of usability of the system interfaces through a quantitative approach. The advantage of obtaining a numeric value about the ease of use is the possibility to perform comparisons between different design proposals and determine the best alternative for a specific scenario. Likewise, a quantifying process of the level of usability provide companies with the necessary mechanism to establish the degree in which their web applications are more usable regarding other accessible websites in a highly competitive market. In this paper, we present the results of applying a variant to the traditional heuristic evaluation process to determine the usability degree of two web applications. This case study is intended to serve as a framework for specialists in this field that are interested in retrieving quantitative data in addition to the traditional usability issues that are identified.