What is JotForm?
JotForm is a free online form builder which helps you create online forms without writing a single line of code. No sign-up required.
At JotForm, we want to make sure that you’re getting the online form builder help that you need. Our friendly customer support team is available 24/7.
We believe that if one user has a question, there could be more users who may have the same question. This is why many of our support forum threads are public and available to be searched and viewed. If you’d like help immediately, feel free to search for a similar question, or submit your question or concern.
This is possible by creating a form calculation and payment option.
Here's how to do it:
1. Create a dropdown box
2. Add a calculation value
3. Create a form calculation widget.
4. Create a condition to update form field value then add the value of the dropdowns.
5. Add payment option. You can review this guide on how to pass calculation value to a field https://www.jotform.com/help/275-How-to-pass-a-calculation-to-a-payment-field
6. To add a tax please view this guide https://www.jotform.com/help/296-How-to-Add-Tax-to-Your-Payment-Form.