Well, you're paying for the middle man. They setup and main the infrastructure for persons that are not technically inclined and for whom it's not feasible to hire a coder to do it for them.
It costs them the same to deliver a $5 file as it does a $50, apart from the transaction fees claimed by their upstream payment processor.
How would you like them to charge for a service like this?
So I would imagine they could pass on the percent component of cost with no markup, and then charge a flat fee (ex: $1). I'm assuming they are never stuck with the bill in a fraud case. If they are, then the price should include that risk component as a percentage as well.
The current model makes a lot of sense for small to medium amount transactions but I too have a problem with 7.6% total of a much larger transaction this company didn't do any extra work for.
Alternatively you could have multiple rates based on the size of the payload. If you're really using their storage, which I doubt, then that percentage cost would cover the bandwidth to serve it. With that you would have people like me that would check their work though, to make sure if I'm really paying bandwidth costs, you better not be leveraging Dropbox's infrastructure for that too.
Personally, I do like the idea over a Zapier though that's a flat monthly fee. I couldn't possibly determine if the monthly rate is enough on principle but a cut of my profits is much less of an "up front" cost I can't adequately measure yet.
An easy system to post ebooks and charge for them is probably worth more to a person selling $200 ebooks rather than $100 ebooks.