On August 30, 2020, Sunday's Post, Tokyo FM's radio show owned by Japan Post Service, announced a collaborative contest with Yoasobi called Letter Song Project to perform a song based on a letter written by listener about thoughts or expectations under the theme "thank you".

[1][2] The winner was announced on July 25, 2021, as sixth-grade student Hatsune's letter, titled Ongaku-san e (音楽さんへ, lit.

"Loveletter" featured brass band club from Osaka Tōin Senior High School, who also co-starred in their online concert, Sing Your World.

[13] For Billboard Japan Hot 100, "Loveletter" opened at number four, behind SixTones's "Mascara", BTS' "Butter" and "Permission to Dance".

[20] Yoasobi gave a debut performance of "Loveletter" on music show Songs on December 2, 2021, alongside "Taishō Roman", and "Tsubame".