FedEx to Britney: Get Help, Biatch!

Fed(ex) up.

That's what Kevin Federline is with his estranged wife, according to TMZ and a number of other sources who say the aspiring rapper is worried and pissed off about her hard-partying ways.

Now even he's telling Britney Spears to check her sometimes-covered ass into rehab, STAT - if only for the sake of their kids.

According to both Star magazine and MSNBC, Federline has tried to work "every angle" to get the commando queen to chill the f*%k out and get some help, but it clearly hasn't helped.

First, he threatened taking the kids away via his lawyer. When that didn't work and he had to settle for a temporary custody deal, he resorted to begging.

Finally, Federline even tried to get his mother-in-law, Lynne Spears, to join the fight against the hard-drinking crotch shot crusader. No word on whether he has touched base with Pamela Anderson about making a special plea to Spears.

"He's afraid she's going to hurt herself," says a friend.

For K-Fed, apparently, it's now all about Sean Preston and Jayden James.

"They're being raised by strangers," said the source.

Amazing that, for all the bashing of K-Fed we do, he's sure looking like the better half of Spederline these days. While Britney's out partying all night with new man Isaac Cohen and pounding Red Bull to stay awake by day (below, left), K-Fed is just doing his thing, starring in a new commercial and on the WWE as a recurring character / nuisance. He even shows concern for his kids. Stunning.