Prepopulate payment form with amount owed and make it as a read only?

We want to use Jotform on our website to collect one-off payments from our clients. We fill in the information on the form, send them the URL where our form is embedded, with the information about the deal prepopulated. But however, we do not want our client to be able to change the invoice/form payment amount, etc. 

Is this possible, and what are the steps needed to achieve this? I do not want to see an example, I'd like to see a resource document or step by step process to achieve this.

Thank you

I think you can achieve this by manually getting the amount from a read-only field. Please do the following:

1. Add a short text entry field in the form and add a default value in it. Make this field as read-only. You can find these options in the Advanced property of the field:

2. Get the Total by calculation widget (https://www.jotform.com/help/259-How-to-perform-calculation-in-the-form).

3. Add a payment field and pass the calculation to it. Here's how: https://www.jotform.com/help/275-How-to-pass-a-calculation-to-a-payment-field.

Here's a demo form, although you don't like it, it should help you get the idea: https://www.jotform.com/72227164508960.