How to add PayPal payment link without set payment amount

We have  a form where 3 variables "decide" the payment amount. We therefore cannot set incremental payments up. Payments could vary from $5 (minimum) up.

How do I do this?

You can actually pass a calculation value into the payment field. Here's a guide: How-to-Pass-a-Calculation-to-a-Payment-Field

You can also create a calculation outside the payment field and then pass the total amount to the payment field. In this way, you have a control over the field values you want to use in the payment field. You may refer to these guides:

How-to-Perform-Form-Calculation-Using-a-Widget
How-to-Assign-Calculation-Value
How-to-Insert-Text-or-Calculation-into-a-Field-Using-Conditional-Logic
How-to-Pass-a-Calculation-to-a-Payment-Field

If this is not what you are referring to, then please let us know and provide more details about the issue. Thank you.