When developing an E-commerce site you will work a lot designing the interface and all the elements you want, and in that process you will have to interact with the team of programmers, designers and marketers, among others, so this tool will help you a lot. Balsamiq Mockups is a good tool to create mockups in a matter of minutes and where the whole team can interact at the same time, and just for 79 dollars, cheap and good enough.

Balsamiq Mockups Home | Balsamiq.