A couple of litres ordered in total (3,4)
I believe the answer is:
all told
'in total' is the definition.
(I've seen this before)
'a couple of litres ordered' is the wordplay.
'couple of' shows that letters should be duplicated.
'litres' becomes 'l' (abbreviation for litres).
'ordered' becomes 'told' ('tell' can be a synonym of 'order').
'l' duplicated is 'll'.
'a'+'ll'+'told'='ALL TOLD'
(Other definitions for all told that I've seen before include "sum" , "Everyone informed" , "In summary" , "Taking every one into account" , "Taking everything into account" .)