Distributing Python Program

[T]

I have a Python program (which I've "frozen" via py2exe) that I'd like
to distribute online, but I'm not sure of the steps to take.  My
thoughts were to create an account with RegNow or FastSpring, who
could accept the payment and offer the download, but I'm unsure as to
how you deal with licensing issues (i.e. how do you implement license
keys)?  I'm only looking to sell the program for around $40, so this
is by no means a huge expense to a business - however, I'd like to at
least have something in place that would make it a bit more difficult
for the software to be pirated (or used above than the paid number of
licenses).  Thanks in advance.