The author is charged the delivery cost whether the delivery happens over 3G or Wifi.
Personally, I doubt the cost to Amazon to deliver the average e-book is anywhere near their quoted rate. If they wanted to charge separately for 3G or wifi delivery I'm sure they could do so: they have simply chosen not to. Possibly it's simply inertia: the original Kindles were 3G only IIRC, so they would have had to pay steep costs themselves for data delivery back then for all downloads. Now they've discovered that authors are willing to eat those charges, why change them?
The consequence of course is that it's uneconomic to sell image-heavy works through the Kindle platform, despite the move to wifi delivery & full-colour tablet Kindles which could exploit them to their full potential. Even books like the OPs find a huge chunk of their profit margin swallowed up by these delivery charges.
Personally, I doubt the cost to Amazon to deliver the average e-book is anywhere near their quoted rate. If they wanted to charge separately for 3G or wifi delivery I'm sure they could do so: they have simply chosen not to. Possibly it's simply inertia: the original Kindles were 3G only IIRC, so they would have had to pay steep costs themselves for data delivery back then for all downloads. Now they've discovered that authors are willing to eat those charges, why change them?
The consequence of course is that it's uneconomic to sell image-heavy works through the Kindle platform, despite the move to wifi delivery & full-colour tablet Kindles which could exploit them to their full potential. Even books like the OPs find a huge chunk of their profit margin swallowed up by these delivery charges.