How do I insert my iframe payment module into the cart and override existing pmt proc

I need a gateway module to integrate Heartland payment processing. The trick is, it's an iframe only system, without API access. I haven't found the tutorial about how to create a payment module so I'm asking here. How do I take the checkout/processing page in zencart and change it to what I need. If I could plug the gateway code into a regular page, the iframe could do it's thing, then I could handle the resulting redirect it gives back and plug that data back into zencart. I'm not seeing where in the docs howto pass the proper variables from what I get back into the cart system.

Any ideas?

I need to override the payment process to use an existing payment gateway through iframes. How would I insert my iframes code into the shopping checkout system?

I've asked how to write a module, but have had zero response. Is there a module howto somewhere. I've searched around and haven't found one.

For coding, I tried inserting my iframe code into the select function from a copy of the authorizenet.php file but the code was rendered in the admin instead of in the cart. I'm not quite sure how the payment module functions render the actual cart a user sees.

Any tutorials or framework documentation pointers would be helpful.

Thanks.

My solution was to have Heartland setup my account to use the Authorize.net gateway. Heartland also has access to the PayPal Pro gateway.

From what they showed me, Auth.net was more economical for less than 2000 transactions per month. Over 2000 transactions and then their use of the PayPal Pro gateway became a bit more cost effective.

Ha ha, I suggested exactly that, to use Authorize.net or something already supported in Zencart. I didn't realize Heartland could use another processor. I thought they were the processor. Interesting. Thanks for the info.