perfectly acceptable ecommerce package called interchange which will do everything you require.
Just for the record, I was not saying there's nothing out there, or that off-the-shelf e-commerce "doesn't exist". Infact one of my very first e-commerce ventures used off-the-shelf software from Intershop.
What I was trying to put in no uncertain terms is what you have also hit the nail on the head with...... i.e.
intricacies of programming and managing
If the original poster thinks e-commerce software is like a copy of Microsoft Office that you can buy, install and forget about, he is very much mistaken.
The maintenance requirements are not to be underestimated. Failure to do so will at the very least result in weird and wonderful bugs cropping up .... and at the worst, will result in a hacker exploiting an SQL injection attack or similar, something which is more likely with free or low-cost e-commerce software because they tend to be coded in a very sloppy and haphazard manner where security is the last concern on anyone's mind.
Personally, I think he should focus on his core business and outsource the hassle of implementing and managing the software to someone else. Volusion,Actinic,Clickandbuy, DigitalRiver (also their Share*it division), or many of the other names out there that are commonly used by software developers and other types of companies selling intellectual property for download.