scsg75 is correct on the postal money order thing. If they scam you after cashing that I believe you can file mail fraud charges.
I woudln't bother with signature confirmation unless you're sending a pretty heavy MO as I think that's a semi-pricey service. You could do DC, but if they claim to not get it, you just file a lost money order claim with the post office and if they actually did cash it their ass will be nailed.
If you send cash you have absolutely no way to prove anything. Neither side does. Very bad idea to deal with cash through the mail all the way around.
Anyone that insists on cash (i.e. won't take money orders) through the mail is probably a scammer. Doesn't seem to be the situation here, but something to keep in mind.