It's going to depend on the payment provider and their API. With most of the boomerang gateways we support, each provider has their own return page which performs the order completion logic. Usually the APIs support the passing over of a return URL and then when everything is completed the user is returned there. You should probably create your own page and implement the logic to finalize the order, then pass that over to your provider as long as that is something they support.
<a href="http://www.aspdotnetstorefront.com">Shopping Cart Software</a>