I've shipping a couple things with the spray foam ("Great Stuff Triple Expanding" seems to work best) but I never thought it was worth the time, effort, or the cost of the stuff to begin with. It also doesn't afford a lot of shock protection, IMHO. The pro shipping foam is great, however.
Drain saw of gas and oil, run out the carb. Leave the tank open for a while if you're concerned about fumes. Take off the bar and chain. Brush as much sawdust and crap out of the saw as you can. Wrap the powerhead in a couple of supermarket or trash bags and tape closed. Get a nice, used box (stronger is better but you can use almost anything you want as long as it's not too small) with good clearance all the way around. Pack the saw in tightly with peanuts or newspapers. Wrap the bar and chain in newspaper and put those in on top of the saw. Longer bars should be packing heavily and sent as a second package. Tape the box closed. Double boxing won't hurt if you're shipping something large and heavy like a 111, SP125, etc.
Use USPS if you're only sending it a state or two away. UPS or FedEx Ground (RPS) are the best ways for long distances. Call them up and ask for a pickup if you can wait for them. I usually don't tell them what's in the box because former employees I've talked to said they played forklift polo with anything marked "fragile."
I've found Mailboxes to be a load of lying, cheating, price gouging jerks. You start spending money as soon as you walk in the door.