Britney Spears & K-Fed -- Tax Lien Busting Team

Britney Spears and Kevin Federline still have that spark ... at least when it comes to covering tax bills -- because the exes have successfully paid off their $37,000 tax debt.

TMZ broke the story ... the State of California filed a $37,712 tax lien against the former couple last month for unpaid income tax from 2004 -- the year Brit and K-Fed got married.

But according to new documents, the debt was settled just days after our post.

It's unclear who coughed up the dough -- but we're guessing Brit handled the lion's share ... because she's nice.