The story starts with how Touma and Yui met, and as time went on, seemingly are pining over each other through love letters after they're forced to part ways. Near the end of the advertisement, however, it's revealed that every line of dialogue actually comes from letters written by the public after Morinaga first announced it was discontinuing Lipton Milk Tea back in March 2022. 
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The advertisement is directed by Toshitaka Shinoda made for Morinaga Milk Industry, advertising the return of Lipton Milk Tea in carton form.<br><br>
(Source: Anime News Network, edited)

667-tsuu no Love Letter