Recently i had to cater a special requirement from a client.
Basically he wanted to capture quite a lot of extra information about the order from the customer and pass over that information to PayPal which in turn should send that info to the customer as well as to my client.

I knew that PayPal would not allow a site to POST any amount of info , but due to client request studied the API in more detail.

In most cases a site would pass item_name_x,amount_x,quantity_x.
where "x" stands for 1,2,3... basically the item number. and PayPal would nicely display a cart of those items ,calcualte the totals and display. and you can send shipping charges also using shipping_1 variable. Otherwise you can let PayPal do the shipping calculation for you.

Recently i integrated moneybooker as a payment gateway to a client project. It was my first experience with moneybooker. If you do not know what moneybooker is they serve the same purpose as paypal and they are being more and more popular.
http://www.moneybookers.com/

I made the mistake of completely following the HTML code generated at
http://www.moneybookers.com/app/test_payment.pl
and then customizing it.

I was able to create a mini shopping cart at the moneybooker side with the few available variables. during testing suddenly when the number of items increased the request to moneybooker from the client site failed.

Finally i was able to enable the Flickr integration in my site. I wrote the code for this about one year back and worked fine in my local machine. But the hosting service did not have PHP5 and PHP CURL which was required for the code i wrote. Actually i did not have to write much code thanks to flickr_api.php Rasmus has donated.
http://toys.lerdorf.com/archives/34-Flickr-API-Fun.html

Any way many thanks to my host for moving to PHP 5 and enabling PHP CURL for me. N