I don't spoon feed, but I will point ppl in the right direction....
with the HTTP debugger (eg. Firefox Tamper Data, Fiddler for IE, webscarab, etc)
you can edit the request before it is sent to the server...if you look at the requests, you will see that for some companies (specifically the ones listed) they send the price as part of the request.
The reason this works better than just messing with the PayPal amount is that some of the automated systems record the shopping cart and then compare the shopping cart amount to the paypal details (automatically).
If you muck with the paypal value, they won't match (some of the ebook sites do this as well)
however, since you are instead mucking with the actual values being sent to the shopping cart everything lines up...the shopping cart really thinks that is the price for the gold.
The "correct" way for the sites to handle this, is for you to order an "item" and then on the back end the server knows the price for the item, then you don't have the option of changing things....that is the way it is done by a majority of the sites.
Basically, get a debugger, and start playing with it....send requests to basic sites (like google) and muck with the request....debuggers usually work in 2 modes, monitor mode, and intercept mode....in monitor mode, there is no interaction, but you can review all requests that have been sent to the server and look at the values. In intercept mode (it is called something different in each debugger) it will give you the ability to change the request before it is sent to the server.....that is how you modify prices.
Once you get the hang of it, start keeping your eye on things that have shopping carts...you will be amazed at how much fun you can have