The song that Donna (Retta) is singing as Ben and Leslie walk into their wedding is a selection from "The Flower Duet" aria from the 1883 opera Lakmé by Léo Delibes. The aria has been used prominently in movies and on television before, especially during the love scene between Susan Sarandon and Catherine Deneuve in the 1983 vampire movie The Hunger (1983) and in a long-running series of TV advertisements for British Airways.
When Ron offers to help with rings he's standing in the doorway in an iconic James Bond pose.