Grimwald
I have an online site that is focused on Tanzanite gemstones TanzaniteAmerica.com. I do not have a shopping cart because I find I comunicate with every potential Tanzanite purchaser. Payment arrangements are made via email or phone calls. A few months ago I decided to accept bitcoin. The single most important thing for me was to find the code needed to convert the set price in USD to bitcoin at the time someone visits the site.

I believe Coinbase.com has utilities/code to do that. I wrote all the code for my site. If you would like my bitcoin conversion code send me a message and I'll send it to you. Not knowing what you have in mind my code may not be usefull to you.