I am on a Joomla 1.5 platform but working to migrate to 2.5 and so purchased and upgraded to Payment Form version 2.3. After doing this, the Joomla 1.5 site always returns 0.00 when someone enters his/her own payment amount. This worked perfectly before my upgrade.
Your documentation states:
Amount : That is the amount users need to pay when processing payment via the form . (I correctly have this set to 0)
Enable enter payment amount : If you set this to Yes , users will be able to enter the amount they want to pay when processing payment. Please note that If you set this to Yes , the amount you set on Amount field will have no effect at all. I DO NOT HAVE AN ENABLE ENTER PAYMENT AMOUNT CHOICE TO SAY YES.
I tried to fake it out by entering 1 in the Form Amount field and then putting a fee calculation into the text field [FORM_AMOUNT]*[FIELD_VALUE]. That works, but the confirmation page adds 1 to the amount rather than multiplies. 300*1 = 300 but the confirmation page says $301.