In what could be considered either a refreshing or a depressing birthday celebration, Jamie Lynn Spears went decidedly down-market to ring in the big 1-7 on Friday, reportedly browsing Wal-Mart and noshing at Ruby Tuesday with rumored fiancé Casey Aldridge.

"They both seemed really happy, both in really good moods," an eyewitness who saw the soon-to-be parents out and about near Casey's Mississippi pad tells People. "Jamie Lynn had a big smile on her face."

Over dinner, "It was just the two of them," adds the spy. "They were like a little adult couple, very low-key."

Jamie Lynn, who is currently sporting a telltale sparkler on her all too youthful finger, later checked out sleeping bags at Wal-Mart with the lately little-seen Aldridge, 18.

Jamie Lynn Spears and Casey Aldridge, 2008

The night before was equally economical as the teenage flames browsed the reasonably priced wares at a local J.C. Penney.

While the stork-awaiting "Zoey 101" starlet spent her big day like the typical teen she isn't, her family -- big sister Britney, parents Jamie and Lynne and brother Bryan -- was chowing down at an Italian eatery in Los Angeles with several friends.

A big-eared mole tells Us that Jamie Lynn's birthday didn't come up in conversation but says her under-parental-guidance sibling appeared to have a good time.

"Britney was beautiful and happy," relays the spy, adding that she was "very polite and composed" during the meal. "She was giggling with her friend and having a wonderful time."

And don't worry -- she didn't forget Jamie Lynn's special day. According to People, Brit picked up a ton of swag for her on Thursday at the kitschy clothing mecca that is Ed Hardy, including purses, scarves and T-shirts.

Strangely, it seems Spears also hasn't abandoned the idea of doing a kids' clothing line, a possibility she floated long before she lost custody of sons Sean Preston, 2, and Jayden, 19 months.

An Ed Hardy rep tells the mag she met with designer Christian Audigier to discuss the harebrained scheme.

You'll recall that Spears previously mentioned doing a tots tog collection during her disastrous, beginning-of-the-downward-spiral interview with Matt Lauer in 2006.

"I have a baby clothes line that's supposed to be coming out. And it's called 'Babies Are Rock and Roll,'" she explained at the time. "There's two separate categories. There's the more elite line and there's the rock and roll part, too."

Lauer then tossed her a softball, asking, "You like business, don't you?" prompting Britney to laugh, "Uh-huh. Yeah. I like money."

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