May 22, 2007 06:00 PM

Britney Spears arrived in Miami on Sunday for a concert at Mansion – but she had some important business to attend to first.

Not 10 minutes after checking into the Four Seasons Hotel in downtown Miami, Spears, 25, and a group of friends zoomed off for a whirlwind shopping spree in South Beach. Their first stop: A 15-minute visit to No Lines Tanning.

Next up: the Guess? store on Collins Avenue, where Spears took shorts, jeans and tops into a dressing room. “She bought quite a few things,” an observer tells PEOPLE. “She left out the back of the store – she seemed happy.”

The retail therapy continued at The Mosh Pit. “They asked if she wanted the store closed, and Britney told them she was okay and not to worry about it,” says a source. Spears tried on several micro-mini skirts and bought a $180 hand-painted pink corset by Kimikal. “She liked it, bought it and wore it out of the store,” says the source.

By 7 p.m. it was time for a sound check at Mansion, where Spears’s dressing room was, per her request, already stocked with Diet Cokes, sugar-free Red Bull, apple juice, a fruit-and-cookie plate, vanilla-scented candles, white roses and pictures of her two sons: Sean Preston, 1, and Jayden James, 8 months.

As concertgoers got out of the rain and into Mansion, Spears ducked out the club just before 9 p.m. and hit The Atrium Miami Beach for some last-minute shopping. With her assistant, she picked out dresses, shoes, belts and accessories, spending well over $1,500, according to a source. She then changed into a pink dress she’d just purchased before heading back to Mansion, where she took the stage at 10:30.

After her concert, Spears headed to the Raleigh Hotel for an intimate poolside after party. But that wasn’t the last stop of the evening – shortly after 3 a.m., she and her entourage arrived at the nightclub SET, where she danced to songs by Fergie and Nelly Furtado before finally calling it a night at around 4:30 in the morning.

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