I would like to point out that you DO get a receipt when you purchase stuff from Steam which you can print/screenshot&save. And it can also be seen in your bank statement.
Steam has really been improved since the early days and it is quite good today.
I play DoD:S and Defcon (This is made of pure golden pwnage!) on Steam.
And when I get my next paycheck I'm going to buy the Eidos gamepack, Shadowgrounds and the SH package.
Already have SH4 and SH3 on DVD discs but I can't help myself because Steam provides a reliable, working and no hassle service for buying games (IMO! Please don't smack me too badly for this pretty please).