Sum values in Column A, based on dates in Column B & if value in Column C = True

I did try to reformat cells but it half worked. Please find WB attached. I am trying to fill in the value in C2 on the 'Returns' sheet based on the Start and End Dates in A2 and B2. i.e. I want the sum of all invoices paid within that period.

PS, it seems to have worked now that I have removed a lot of info from the sheet however certain values are still FALSE.