Hi David,
You could set up a simple password to user with API and along with password policy, this could force him to change the password.
Or you could after purchase re-direct him to a page which also contains account management web part along with warning about account password.
Last thing that I can think of, would be to use password expiration. You could set up user password with API and have it expire in next 24hours, so that on next day, he will be forced to change it.