Retailer's exchanges left in envelopes to be returned (5)
I believe the answer is:
sales
'retailer's exchanges' is the definition.
'sales' can be an answer for 'exchanges' (I have seen 'commercial exchanges' mean 'sales' so perhaps 'exchanges' could also mean 'sales'). I am not sure about the 'retailer's' bit.
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for sales that I've seen before include "a feature of Boxing Day?" , "Marketing" , "Cheap offer periods" , "Public auctions" , "Periods when shops sell goods at discounted prices, bargain times" .)