'A' modifies 'pair', so the structure is fine (and common). While I see your concern, your alternative actually reads as less felicitious (too many 's's).
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Mister Micawber'A' modifies 'pairSorry, are you referring to the article before sunglasses and saying that it modifies 'pair' (as well as sunglasses), which makes 'pair' singular?
English 1b3 it modifies 'pair' (as well as sunglasses)'A' does not modify 'sunglasses'; it modifies 'pair'.