I'm encountering a slight problem. How does one embed a payment page from Paypal or Google Checkout into a website that does not use a site builder, just raw source code? The explanations given on the respective websites don't quite help me.

I have a donation button on my site, and Cemetech does to on the jsTIfied page. But for a cart based system see:

https://www.paypal.com/cgi-bin/webscr?cmd=_shoppingcart-intro-outside
http://checkout.google.com/seller/integrate_shoppingcart.html

If you wanted to do more of it "in house" you could create a script that saves the client selections then creates a button for PayPal & Google Checkout for the total amount due. Once the client lands back on your site the order receipt will be sent through to the vendor for processing.