Here is how you can allow users to specify the amount of money they want to pay or donate on your online form:
1. Create a Number field in your Form Editor for your form users to add the amount they want to pay or donate for your services. Use this field to make sure users won’t submit invalid values(e.g. characters or symbols) which will not be recognized by your payment processor .
2. Then, in the Settings → Payments section of your form, go to Summary and Discounts and create a calculation formula by multiplying the Number field with 1 (from the Quantity column). For example:Amount X 1.
3. Save the settings. Now, the amount charged will be the one specified by users in the Number field.