I suppose the payment gateway does let you choose on or off site, right?
Wrong. It is the choice (the type) of payment gateway which decides whether the processing is done on-site or off-site.
For instance, if you were UK based and chose "Sage Pay Form" as your payment method then the customer will be taken to the Sage Pay website and make their payment there. If you chose to use "Sage Pay Direct" or "Sage Pay Server" then the customer will remain on your website, enter their credit card details and the data will be sent to Sage Pay, from there to your Bank, back to Sage Pay and back to your website but all in the background whilst the customer remains on your website.
Vger