Incremental pricing fault at checkout

Urgent help gratefully received! We have a form for script submissions… When an entrant wants to submit a script over the standard 120 pages, they need to get charged $2 per page over 120. This has been successfully set up within the form using merge tags and the total cost is calculated correctly at the bottom of the form for the customer BUT when the payment is then produced, this particular art of the pricing IS NOT CHARGED (but the rest of the standard services / add ons are processed - it’s only this one that doesn’t). Gravity Wiz have been very helpful and logged into our back end and run tests and replicated this fault but are also confused by it and how/why it’s happening. They say it’s a Gravity Forms issue somewhere. I have raised it with GF last week but received nothing back from anyone yet. I’ve chased them with no effect. Getting desperate now, because this is obviously having an impact on business. Please help! Thank you.