Los Angeles, California - Olivia Rodrigo celebrated the three-year anniversary of her acclaimed debut, SOUR, by treating fans to a special – and mysterious – merch drop.

In honor of Tuesday's big anniversary, the 21-year-old singer took to her Instagram story to thank the Livies for their continued support.

"sour turns 3 years old today," she wrote. "forever grateful for music and the way it brings ppl together.

"lots of love in ma heart for everyone that has listened and supported. nostalgic af."

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The post included an old photo of Olivia posing in front of a large billboard promoting the album.

Along with the tribute, the deja vu artist also added several new merch items to her online store – several of which were kept a mystery!

Fans were given the option to purchase a mystery tee, fleece, or accessory.

The fleece could be either a hoodie or sweatpants, while the accessory boasted three different options that were not revealed – though the photo depicted a bucket hat and a pair of socks.

The mystery merch line has since sold out.

SOUR's big birthday comes just days after the Grammy-winning record became Spotify's most-streamed album by a female artist.