Cross-site and cross-platform form submission

I have been tasked with sending form details to another domain. We are a .NET shop so all of our forms are in ASP.NET/C# and the other domain is a strictly PHP shop.

On smaller sites I’ll often just send an AJAX post method using jQuery, but I am a bit wary of doing this on a large website where users may not have JavaScript enabled and each form submission is worth money to us. It is very important that every single submission sends perfectly without error.

Note that I do not have control over the other domain. I am basically sending to another page that I cannot control much.

What is the best possible way of doing this from a ASP.NET site to be received by a PHP site?

If you need strict control, I think SOAP or newer could be the answer.

You would still need files on the other side.

Off Topic:

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Well there are some questions that need to be asked first. The first of which being how will possible form level errors be handled in regards to the site making the request. The other is whether it is absolutely critical that the form submission be masked. This other site that the request is being sent to must already have a form or no? If it does then are you just trying to avoid using an iframe or is there more to it (more to it I suppose).

It feels dirty to say this, but at the moment we are using an iframe. The reason we want to send to another domain through our form is to do with one of our affiliate tracking scripts that run on our thank you pages (which naturally only appear when a form has been filled successfully). These need to take information from the forms, so we can’t just redirect from the iframe.

The idea was to send to the form on their site through one of our ASP.NET forms. The only solution I see is using a AJAX post request, but I am worried about the robustness of that solution.