How can I include product dimensions in my listings?

It would be very easy to add. I could be wrong about this but I assume that Shopify gets a cut of any aftermarket apps you buy through their App Store. There are aftermarket apps that will make these calculations for you. But there is a monthly fee. The effect then is to significantly increase the actual annual cost of your Shopify store but leaves Shopify able to still advertise a relatively low monthly fee. Please somebody in an official capacity jump in here and tell me I’m wrong.