Buying additional items?

I'm a long time customer and fan of Simplisafe, but it seems so difficult to go online and buy additional items like sensors, batteries, etc. If I go on my account, there is no where to shop. If I go on the website, there are only whole systems to buy. Why is it so difficult? Seems like there should be a place on my account to shop, since it knows me and my system. Maybe I am just missing something?