Lance Bass Picks Up Tab After $20,000 Birthday Weekend

Lance Bass celebrated his birthday a week early this year -- like a BOSS -- treating himself and several friends to a lavish, alcohol-fueled trip to the Caribbean ... and TMZ has learned Lance paid for the entire thing.

A rep for Lance tells TMZ, the former *NSYNC singer flew his boyfriend (above) and 10 friends to St. Martin from last Thursday to Sunday -- and stayed in a luxury villa at the island's La Samanna Resort & Hotel that ran $4,000-a-night.

Lance also sprung for a day of jet skiing at $100/hour ... got a private V.I.P. table Saturday night at Tantra nightclub.

We're told the final bill totaled $20,000 -- but the hotel stepped in and comped a bunch of stuff, including the hotel room. Still, Lance sprung to cover the balance ... which wasn't too shabby ... and his friends didn't have to pay a dime.