You don't need one of the special boxes; the plastic bags work. I did this about a year ago and I got the results within a week after mailing them to auburn.

There should be instructions online telling you exactly what to do and how to bag it.

In answer to your original question there is also Waters Laboratories in GA that I’ve used before.

257 recommends anyone other than Auburn (and he is an Auburn fan). He feels they haven’t kept up with changing practices and theory (or something along those lines).

Paper bags versus ziplocks isn’t a big deal unless some of your soil is wet. Stop by the Extension Office and get the paper bags if you’re sending them to Auburn. Go to a local chemical dealer (Helena Chemical or similar) if you want to send them somewhere else. If you’re not buying the fertilizer from them be prepared to pay for the bags or be told “we just ran out”.

You didn’t ask this either but my routine was to get three samples in each plot about the size of a softball. Put them in a 5 gallon bucket and mix them up good. Then fill up the bag with that and dump the excess. Getting a sample from just one place in each plot runs the risk of that having been a place fertilizer or lime was spilled and getting an analysis that doesn’t represent the dirt across the whole plot.

Also don’t forget to label the samples so you know where is where and what.