Online Purchasing

I have some software I would like to sell online, so that a customer would go in, buy it with credit/visa/master card etc, and be given a special download link to download it.

Couple of questions:
1) What does it take to handle credit card information from users? Are there companies that would handle all the money stuff? If so, how much money would they take from each sale?

2) If I rolled my own, what sort of problems might I face? I am proficient with PHP and .NET, so I could do the web side, but what about the money side? I've never worked with anything like that.

3) I would like to make my own page where the users enter payment details and everything, and then for my page to submit that information to the bank or whatever and have the money transferred into my account.

4) The customers my software is aimed at is other software companies and businesses. Would my sales be lower because I'm making them only purchase via credit card and a download? It will only be like a 10mb download at most.

The sort of system I'm looking to create is this

It is, of course, quite easy to knock together a PHP page to collect necessary information. However, credit/debit card companies have a myriad rules and regulations which you need to follow. It is a total and absolute quagmire in Denmark, I suspect the same is true elewhere in the world.

I ended up buying a "webshop" package which allows me to set up a typical small shop and it handles everything. The web shop package cost 3500 DKK, ~£350, and I pay a fixed monthly fee to "PBS" which is the banks clearing system here of about £15 a month.

This was shown to me by an old friend, http://www.oscommerce.com/. Has anyone here had any experience with it?

www.unitedwebhosting.com

They set it all up for you and everything is secure.