Dyanamic value

Hi
I have used this plugin.

But it accepts fix amount for the transaction which is fixed by admin from the backend in free and paid version.

My website has a bill payment process.
Where users come and fill in all detail then send the form.
Then enter the bill amount then pay a bill.

I need this type of flow where the user adds an amount.

The page I need help with: [log in to see the link]