Are the continual pop-ups to upgrade to Professional an active policy to infuriate users? Back around Christmas we had the "Today only" which lasted about a week, several times a day and for the last few days there's been another one a few times a day.
Saying I'll pass or no thanks once should be enough, twice at the most.
If anything, in my case it makes me even more determined to not upgrade.
This would most commonly indicate that the cookie storing the information that you've already received these messages recently is being cleaned, thus preventing the software from knowing that these messages have been displayed to you.
I've included a small text file of the sites that should be present in CCleaner's Options > Cookies > Cookies to Keep list to prevent this from occurring.
The list is as follows:
*.avast.com
*.ccleaner.com
*.ccleanercloud.com
*.piriform.com
Please also make sure that other programs aren't cleaning these cookies (such as a browser set to clean cookies automatically on close).
With the appropriate cookie(s) in place, the software should be able to keep from overmessaging you. (For instance, the 'Boxing Day' sale you mention was intended to show one message only.)