As one chooses a checkout with divides (2,4)
I believe the answer is:
at will
'as one chooses' is the definition.
(synonyms)
'a checkout with divides' is the wordplay.
'checkout' becomes 'till' (a supermarket checkout).
'with' becomes 'w' (abbreviation).
'divides' indicates putting letters inside (inserted letters create a division or opening).
'till' placed around 'w' is 'twill'.
'a'+'twill'='AT WILL'
(Other definitions for at will that I've seen before include "Whenever one pleases, say fire rifle" , "take it as I please" , "when one wants" , "Whenever you please, as you like" , "When I choose" .)