I'm creating a form and would like to add an option for the user to submit payment. I'm not aware of this existing in Marketo functionality but are there any workarounds or plug-ins?
Yes, it is possible to use (for one example I've already used in prod) the Chargify API together with webhooks to ensure that a lead has paid for a product or service before taking further action in Marketo.
Whether Marketo is the best point of entry for such stuff is majorly debatable.
Just wanted confirmation that there is no out-of-the-box functionality (in Marketo) that lets someone submit a payment via a Marketo form and that this is really only doable using a third party service in combination and the webhooks tool? Also, why is it "majorly debatable?"
There's no out-of-the-box function.
That said, if the form is used to transport information to another system, and that other system has a secure login, then the whole thing can be made to work. It's just a very, very uncommon application of Marketo forms (then again, most people don't push forms as far as they could/should).