THE Gastineau Girls are already an hour late.

Brittny and Lisa, the mother/daughter drama duo on E!’s reality TV show “The Gastineau Girls,” were supposed to be downtown at Century 21 at 11 a.m.

Apparently, the traffic from their Upper East Side apartment-in-the-sky is a lot worse than they expected.

When they finally arrive in a taxi, Lisa is full of energy and Brittny looks slightly irritated. There are no apologies. Only meet and greets.

One can assume that Brittny is mad because The Post is putting her to work.

We challenged the estranged daughter of NFL star Mark Gastineau to find an outfit for Lisa for Mother’s Day – with only $100.

And she’s never worked a day in her life.

“This is quite the challenge,” says Brittny, 22, whose mane-like hair complements perfectly with her boho chic look: white peasant skirt, chunky belt and cowboy boots.

Lisa is wearing a Diane Von Furstenberg wrap dress and gold Dolce & Gabbana shoes, along with a Hermes bag and lots of gold jewelry.

“She’s more glamorama than me,” says Brittny.

“Really? Let’s hear this big tell-tale,” says Lisa, in a sarcastic tone that insinuates Brittny is just as high maintenance.

This bickering between the mother-and-daughter team, who act and look more like sisters, is what the cameras capture on their reality show (which is in negotiations for a second season).The women have a lot in common (University of Alabama grads, aspiring single models, devoted shopaholics), but they also have different styles.

Let’s see if Brittny can dress her mom for Mother’s Day. And if Lisa can resist putting in her two cents.

“I’m thinking about white pants, sandals, and a cool belt,” says Brittny, who plans on spending Mother’s Day in the Hamptons with Lisa. “It’s the one day to thank her for everything.”

Century 21

22 Cortlandt St.; (212) 227-9092

There really is everything you need in this endless emporium of shoes, designer clothes and accessories galore.

It’s no wonder it was voted the No.1 discount store in New York City in the Zagat Survey shopping guide.

We start in the junior’s department to get more bang for our buck.

“Can you hold on to this for now?” asks Brittney, who gives me back the $100 I gave her. “I don’t want to lose it.” That’s a first.

There are peasant skirts, lace tops, capri jeans and tank tops everywhere.

A woman, who recognizes Lisa, says: “Don’t share. Don’t let out the secret.”

“Everyone knows about C21,” Lisa says. “It’s no secret.”

Brittny pulls a peacock-print skirt from the hanger, and shows it to her mom.

“Are we going for the Big Bird look?” Lisa says. “OK, that goes in the ‘almost Cavalli’ category.” Brittny puts it down.

But she does grab several white pants, lace tops and a gorgeous, chunky Moschino belt that her mom approves of.

We go upstairs, and it’s trend central, with hip clothes from Juicy Couture, Le Tigre, James Perse, Nanette Lepore and Rampage. In the back, there are racks of high-end couture with designers like Chanel, Missoni, Versace and Alexander McQueen.

Brittny has an epiphany.

“What about a button-down collared shirt, white pants and flip flops?” she asks.

“Only 3/4 length sleeves,” warns Lisa, who is sifting through the Pucci dresses and tops.

Brittny finds her magic item: a gorgeous blue Michael Kors tunic, and it’s within the budget. Perfect. All we need now is shoes.

As we head down the escalator, Lisa (a former retail chain store owner) is lost in shopping land. Brittny has to peel her away from the Christian Dior section.

On the shoe floor, they carry everything from Frye boots to Ralph Lauren heels to Jimmy Choo stilettos. But Brittny likes a pair of silver Nine West sandals.

She has nearly everything except for a belt, which is necessary, because Lisa doesn’t want to look like a whale.

“I didn’t get a little waist for nothing at this point,” jokes Lisa (but not really joking).

Realizing there’s no more money left for the belt, Lisa must use the dark blue belt provided by the “Tuff Babe”-brand jeans.

With the outfit complete, Lisa emerges from the dressing room looking like she’s about to set sail on the Caribbean Sea.

“This is how I want you to look on Mother’s Day,” Brittny says. “I love the silver and turquoise.”

Lisa agrees: “I’m walking out in this outfit. I love it.”

“This is the first Mother’s Day that Brittny’s ever gotten me a gift!” she says. “Thank you New York Post.”

“Oh my god, I can’t believe you just said that,” says Brittny. “I’ve gotten you Mother’s Day presents before.”

“When?” asks Lisa.

“Like, every Mother’s Day. Every year,” says Brittny. “Maybe with your credit card … .”

End of argument. It’s time to leave, and the girls have a meeting that they are an hour late for (that’s no surprise).

Silver sandals with turquoise stones, $49.97

Low rise Capri pants, $17.97

Blue designer tunic, $34.97 (reduced from $89)

Total cost: $102.91