How does O. Henry’s use of irony create a humorous tone?

My favorite use of irony in "The Ransom of Red Chief" is the verbal irony that happens at the end of Bill and Sam's letter to Ebenezer Dorset.  

These terms are final, and if you do not accede to them no further communication will be attempted.

Two Desperate Men.

Remember that Bill and Sam are two "hardened" criminals that have kidnapped a child.  They are dangerous men presumably because they are desperate enough to try anything . . . including kidnapping. At least that's the message that would be sent if the kidnappers were anybody other than Bill and Sam and the victim was anybody other than Johnny Dorset.  In this case, Bill and Sam are desperate to get rid of Johnny.  They are at their wits end.  They are scared of Johnny and desperate to escape his enthusiasm.  

The other irony that is humorous to me is situational irony.  I mentioned before that Bill and Sam are criminals that are willing to kidnap a young boy.  They should be cruel enough to control and scare little Johnny Dorset.  That's what readers would expect to happen; however, the absolute opposite is what happens. Bill and Sam actually cower from Johnny and have zero control over this kid.  Ironically enough, they don't get any ransom money.  In fact Bill and Sam end up paying Ebenzer to take his kid back.  So for all of their trouble, Bill and Sam actually lost money.  That story is hilarious, because everything that happens is ironic.