Select the “Settings” tab of your charity account and click on "Billing". Click on "Resend" next to the required invoice to have it sent again via email to your inbox.
Note: Since August 2020 GivenGain has stopped billing charities that were making use of the paid Premium option. All premium features – including unlimited fundraising campaigns, analytics, custom domains and third-party integrations – are free to use for all charities on GivenGain.