Inna propozycja, bioraca sie z tego, iz:

lump sum = A single sum of money that serves as complete payment (def. Google)

a wiec jest to platnosc nie tyle zryczaltowana co jednorazowa, w przeciwienstwie do flat fee. Jezeli np. dziennikarz jest na ryczalcie, to nie znaczy, ze dostaje co miesiac jednorazowa "lump sum", ale stale te sama "flat fee".

... Remuneration
will be on the basis of a flat fee, travel and telephone costs. ...
mailman.greennet.org.uk/public/creativx-l/ 2002-April/000067.html

Experience has taught me that advertisers get the best
results when they pay their agency a flat fee.
www.ad-mad.co.uk/quotes/remuneration.htm

For any item the publisher can choose to charge the "utilisation"
fee on a "per-student" basis or a "flat fee" basis. ...
www.leeds.ac.uk/library/rights/licences/cladigi.htm