Let's assume you adhere to the Oxford-comma style and write American English. Then, beyond any reasonable doubt, the sentence
A. Mary bought fish, meat (which smelled rotten), and bread.
Now consider the sentences
B. Mary bought fish, meat (both of which smelled rotten), and bread.
C. Mary bought fish and meat (both of which smelled rotten), and bread.
Where to put the commas in these two sentences and why? Do you prefer B or C (and why)?
The sentences B and C present a particular difficulty to me, since the scope of the parenthetical remark extends not only over the second item (as in A, where we have a simple list of three items) but also over the first item of the list (so what we have is probably not a simple list of three items).