@turkay1-- You're right. There are very good reasons for having invoices online. Some stores even request a copy of the invoice when you are returning a product for a refund or when you're trying to have it replaced.
I bought a DVD player online last month and it was not working properly. I called the store and they asked me to mail it back to them with a copy of my invoice inside for the refund. The invoice was in my email and I hadn't even looked at it actually. But it was so easy to pull it up and print the invoice out.
If it had been in paper form (and some online stores also include a paper invoice in the product), I would have surely lost it. And if I didn't lose it, I'd still have to go and get a copy of it. It's just too much work that way.