First of all, I need to apologize for not replying to your post sooner, I thought I already had. Long story short, the current version of Method likely won't be able to do what you are asking here. However, I believe what you want could potentially be done through the MethodAPI, but this would involve building a custom application that would communicate with Method. If this might be of interest to you, you might want to check out this page from our Website, you will find more information regarding the MethodAPI there. We also have registered Method Solution providers that have experience working with the MethodAPI, you can find the list here.
Although I can't give you an ETA at the moment, our development team is working on a version 2 of Method. One item we are planning on offering in this new version is the ability to make certain screens in Method available publicly. With this ability, your form can be much more dynamic, it will be able to query/communicate with your Method database during the form submission. Also, because of this, you will be able to do customization like your example. You should be able to create the form, then have a check so if someone has paid, an invoice could be automatically created. I believe too, if an invoice is created for a new Lead, it will automatically convert them to a Customer.
You asked about webhooks, I don't believe Method currently uses them, but please understand although I understand the concept of webhooks, I am not exactly sure how they get implemented. If you want some specifics on this, I would suggest posting in our MethodAPI forums, our developers maintain this section and would have some better insight.
Again Alfredo, I am sorry for not responding to your post earlier, but hopefully you will find this information helpful.