I will copy/paste the paragraph from my comment that you seem to have ignored :( where I already explicitly listed the things that Apple does which, for example, PayPal does not:
"handles all of your complex accounting burden, takes care of district-specific sales taxes, manages currency normalization for worldwide distribution, and deals with any and all payment-related support requests (in numerous languages)"
The App Store is a retail store: you hand them product and give them a general price point, and they handle the rest. All you need to do is develop your product and (hopefully) handle support directly related to the product itself. Otherwise, your abstraction is you are just sent a check every month.
If you choose to sell a product yourself, I hope you have a good grounding in sales tax law. For an example, did you realize that you cannot legally sell digital products to the EU, no matter what country you are a resident of, without registering for and collecting VAT?
You also will be dealing with a drastically different kind of support request, as there will be people claiming that you stole money from their credit card, that they didn't intend to make purchases, that they thought the price was different, that they made a payment to you with one credit card but now wish they had used a different one... some of these people are lying, some of them had their credit card number stolen, some of them don't realize that a member of their family uses their PayPal account to make purchases online, and all of them are much angrier than your normal support request. You can build systems that make these issues come up less often, but honestly then you end up spending much more of your time on payment processing than your application, so you should just pay someone else to take care of it for you.
