Welcome!
I would have WalMart deliver the boxed bike to your door rather than getting it assembled in-store. There honestly isn't a huge amount different between a $250 Walmart bike and a $300 bike shop bike, but the bike shop bike will be assembled by someone who knows what they're doing - and someone will be there to do a final check to make sure everything is tight, reasonably well adjusted, and in the right position before giving it to you. Local shop should be able to do it for <$100. If you have multiple shops around you, I would pick the one that offers a 30-60 day adjustment guarantee - brake and shift cables stretch a bit when they initially set in, so a lot of shops will be willing to tweak the adjustments - this is generally limited to turning a few barrel adjusters, but if you're new to the sport, it can make the difference between a bike shifting to an easier gear... or not.
One additional thought, if you can swing it - the Decathlon probably comes with wire bead tires, which add loads of rotating mass - won't affect your overall speed, but will definitely make you feel more sluggish pulling away from a stop, or in sudden accelerations (if you're trying to beat a friend in a sprint). If your shop has kevlar bead (also known as folding bead) tires for a reasonable price (say, $40/each or less), might be worth swapping those in for an immediate upgrade - you could save over a pound, make the bike more fun, and likely have more grip for cornering and braking, too. I would suggest taking the bike out for the first month with the OEM tires, then swapping them (maybe with the help of some $5 tire levers and a youtube video) - you'll likely see some substantial improvement!