To help your organization raise more money online, FirstGiving prompts donors to increase the amount of their donations to cover the costs of processing their donations.
The Net Payment Amount aka Donation Amount column in your payment report reflects the donor's intended donation amount. The Payable Amount reflects the amount after fees.
Bear in mind that your donors will receive a receipt email for the gross amount - the amount their card was charged. On the fundraising page where they made their donation, the "intended" amount will be shown - so a donor who gives $50 will be listed as having given $50, regardless of whether they chose to cover the fee or not.