2chekout.com and paysystems.com

Which one of this is more flexible to integrate with customized shopping carts,
Site is not selling web hosting and i do not want to create products on their site , so
my total sales values need to be passed to the payment gateway interface ,

It could be for 1 , 2 or more items,

Can the above payment processors be used for such shopping site,

Please give your comments,

If any one is using this for their site then please mention site name here so that i can check it out.

With regards,