I am working on a custom payment module and I want it to support multi-shipping. Currently I have developed one that works fine, using direct server integration (i.e. the customers are not redirected to some hosted page on the gateway server, a credit card form is displayed on Magento to collect information instead).
But now I need insights on how to make it work with payment gateways that are using hosted pages. I guess I would have to calculate a lump sum for all shipping addresses, and pass this amount to the payment gateway. But then how could I identify and update these orders when the system receives responses from the notify or return callback from the gateway? So far I could not find any similar references.