For Vendors from whom you purchase fuel. Go to Purchasing Module (A/P) then under Maintenance → Vendors

Search the vendor name

Upon double clicking you will open that specific vendor, you need to check Transport Terminal checkbox for that Entity on the Vendor → Detail tab → then hit SAVE

Set up Supply Point. You can get to Supply Point from Vendor (Entity) screen → Locations tab → Vendor Location screen → Supply Point tab.

Select a location under Vendor Locations Tab → Click Open

Details tab → Gross or Net dropdown: select whether this Vendor Location bills by Gross or Net gallons.