Yes, you can do it from the web version of Floodgate or via email.
From the web version of Floodgate, navigate to Settings (cog button on the side bar) and select 'Billing' tab. Refer to the volume discount table there and decide how many credits to add. Please then add the number of credit that you prefer to purchase in 'CREDITS' field and the 'AMOUNT' field is auto populated. Lastly, press 'Add Credits'.
Alternatively, please send an email to firstname.lastname@example.org and let us know how many credits you prefer to add.
In both cases, you will be receiving an invoice from our accounts department shortly after. Please process the invoice accordingly.
You can check remaining credits and the history of your purchases in Settings > Billing.