Sorry for the late reply - I never saw this post until now 

Unfortunately the answer is both yes and no.

Contracts and rates are very seller specific, T&Cs that apply to one will not apply to another, and whether you get a better deal than My Post is really going to depend on the weight / volume of your average package, as well as how many packages you send to what Aus Post consider "rural" areas. 

Some sellers on eParcel have contracts where they are charged on dead weight only, while others will always be charged the higher of the two between dead weight and cubed. Other sellers I know use pre-paid satchels for a lot of their packages, and only use eParcel for large / heavy shipments. 

Some sellers don't have extra cover as standard with their parcel rates, and return post (if the package is RTS for any reason) is more expensive than a regular parcel of that size / weight.

I can tell you only one (possibly helpful) anecdote, which is that one seller I know was advised by Aus Post not to even bother asking about eParcel unless / until they are sending at least 1000 parcels a year (20+ per week), otherwise My Post was their best option.