Amazon - Should I choose the 35% or 70% royalty rate?

There seems such an obvious answer here. If you are selling an ebook for $2.99 then the 35% royalty rate would be $1.04 and the 70% royalty would appear to give you twice that - $2.08

Except it's not quite that simple because Amazon make a delivery charge if you use the 70% rate. They subtract that amount before calculating the 70% royalty.

This delivery charge is based on file size and for a normal book, file size is something like 2 megabytes. The download charge for that would be about 30 cents. That means your 70% royalty rate would give you ($2.99 - $0.30) x 0.7 or $1.88

But suppose you have a book with lots of images? Such a book could have a large download size.
Here's a book like that:

As you can see this book earns a higher royalty by selecting the 35% rate because you are not charged that delivery fee.

So the answer to the question posed is really - Not always.

Other retailers don't make this download charge. Most offer a 50% royalty and that can make a big difference if there is no download charge. Sixty percent of $2.99 is $1.74. That's much better than the $1.05 the above book would get at Amazon. best of the bunch appears to be Smashwords which offers up to 85% royalty rate.