looking a similar payment gatevay module for servired

I would like to develop a payment gateway for Servired. It's a popular gateway payment in Spain. I'm looking for a similar module developed because I'm not very familiar with Drupal's API. I'm going to explain you how Servired works:

When the user clic the final "Place your order" button I must to submit a form with Post method to the Servired Bank URL. The fields are similar those passed with paypal, the final price, the txnid... That's my principal problem now. I have been hacking the paypal module, but they construct one URL that the payment.module finally sends with drupal_goto. I don't know how to send a form if the user doesn't clic a submit button. Do you understand to me?

The web page of the bank ask for the credit card number and if the payment is OK then they send a form to one of my drupal pages with the fields required to register the state of the operation. That's similar to paypal. The bank sent an encripted signature in a field to verify the operation is true and no hacker is trying to confirm operations. I have the functions to verify the signature, so I think it doesn't should be a problem when hacking the paypal module.

I need to develope this module and I would like to publish it as open source. Any help will be appreciated.

Best Regards.