WordPress, Custom Field Suite And The WP REST API As A Middleware Platform !!TOP!!
To be able to work with form builders, you need software serving as a bridge between the two systems (aka middleware or connector). Only if you have one, you will be able to map form fields to respective Salesforce fields via a foolproof graphical user interface. Once you have the necessary middleware add-on, you can enter data into both core and custom Salesforce objects (it is a one-direction procedure where Salesforce is a target) as well as integrate payment processors.
WordPress, Custom Field Suite And The WP REST API As A Middleware Platform
Salesforce is a wonderful platform to integrate to or from. The API is great, there are connectors for several different languages and integration middlewares, and you can create custom APIs in REST or SOAP. Using a proper integration platform such as Boomi, CastIron, Informatica, etc. you can pretty much do any kind of integration your heart desires. However, to keep costs low people often want to just have Salesforce talk directly to the other systems to fetch and save the necessary data, a.k.a. point to point integrations. This is where things can get very tricky.