Happy to see you’ve decided to join our ranks! Welcome, @Luka_Felicijan 
Good question! Our Form Builder app would be a great fit for this use case. Here you can add different field types (from dropdowns to multiple choices) and even set conditional logic for their display:
To redirect your clients to the checkout page, you should choose the Redirect to URL option for the Post-Submit Action on the Settings tab of your widget’s settings and add the needed URL:
There are 6 different ways of receiving the submissions, and this article describes each way in detail - Where to find your Form Builder submissions - Elfsight Help Center.
The only limitation for your use case is passing the submitted form data to the form on the checkout page. Unfortunately, this feature isn’t supported now.
The good news is that we’re currently working on adding the Stripe integration, which enables users to pay directly from the widget. If you’d like to have this feature, please upvote this idea to get notified about the release -Stripe Integration.
If you have any questions left, I am always happy to help 