30 minutes is fine actually. I get fish shipped to me all the time, and when it's done right, the fishes should be fine. Of course, it's about $40 for express 8S
But, look for a fish that is 'active', which is not likely to happen in walmart. A lot of them will be listless and at the bottom, but if you find one that's just giving everyone around him a lot of hell (flaring, swimming around like he's awesome), that's a healthy fish and you should snap him up!
For plants, stay away from the plastic crap. They make silk plants now that are better for the bettas, and prettier. If you can get with flourescent lights, and get an average of 2 watts per gallon, you can use live plants which are SO good. They use up the bad stuff and put out oxygen for you feeshes, and help keep algae down. Good plants for bettas are numerous, just google - I'm sure there's a list.
Walmart actually has 5g and 10g starter kits - they come with the filter, the hood and lights - but I think the lights are incandescent. Which is fine if you want to use silk plants, but the flourescents just give off a cleaner light. The 5g kit is about $30, and the 10g is about $40 - well worth the price, and comes out a little cheaper than buying everything separately. You'll need a gravel vacuum to siphon out the nasty poo($3-9), a thermometer($1-3), food ($1-3), and decorations - which you can go as high or low on those as you want. And get at lease one cave. They seem to love caves.
Gravel is easier to use as a substrate, I prefer sand but it's a pain in the arse to get it clean enough to go into an aquarium! You'll want to cover the bottom with about 2" of gravel, more if you prefer but 2" is a good depth. I can't remember how much it is at walmart.
It'll probably be $60-120 depending on where you get your stuff from. OH AND A HEATER!!! Don't buy the walmart heaters. They SUCK. They'll do in a pinch if it's freezing, but it's worth the money to get a good heater.
This place has amazing prices; That Fish Place