Britney shopping in Miami

Britney Spears arrived in Miami 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.