Hey BG305! 
We have been in discussion about this for some time. We agree selling by weight makes more sense than by volume when considering terps are mixed by weight (by most people). When we first started our company the industry standard was to sell by volume, so we went with that to avoid confusing customers.
We do get quite a few calls per week asking how much X.X mL weighs, so customers can calculate how much volume of product they need to buy. We explain the specific gravity for all products is listed in their respective COAs, and how to find mass from volume using specific gravity. We are also considering updating our mixing calculator to offer volume calculations, but doing so in a non-confusing way is non-trivial.
Long story short, we may switch from selling by volume to selling by mass. But that’s a big change so we want to make sure we consider all pluses and minuses to making the change. We are going to start adding the weight of the terps to each bottle label and product webpage, along with the volume.