Sum ( DONATIONS::Amount )
Define a calculation in the Donors table as: Sum ( Donations::DonationAmount )
Define a summary field in Donations as the "total of" DonationAmount.
Place this summary field from Donations on your Donors layout.
The calculation field will update more cleanly when you edit data in the portal, but the summary field can be used with Filtered portals and with other reports and layouts in addition to the one you describe here.
Thank you so much for helping with this!