Hey there, @Darryl 
Thank you so much for the idea!
Unfortunately, it’s impossible to display the payment button based on the selected option now. However, I agree that this would be an awesome addition and we’ll try to think about it in the future.
As for the conditional logic for the form fields, I’ve reached out to the devs and it seems this feature was overlooked when releasing the form feature. We’ll try to add it in one of the future updates.
We’ll keep you posted in this thread 
- Conditional logic for form fields