This is a serious matter, and I fully understand OP's concerns. In all respect, here are two scenarios that iObit, who's pretending to offer solutions that are part of fighting malware/spamware/spyware/viruses, should, or better said, must be aware of : what it should be; what it otherwise would be.
Since the advent of the Web, experiences of vendors and services alike having activities on the Web along a growing client base came up with ethical behaviors that makes the Web a better environment for everyone, for user as well or as for businesses. Some are not enforced by formal legislations while counting on everyone's good will, however they could become as such if they continue to get ignored.
What it should be
For free product versions, users understand they'd maybe have to endure advertisements for other products. Even from third parties, although this would be a tricky business for the provider as these products are then considered to be "adware" (and to some extent "spyware"). It's normal, they know the developer/vendor should get something in return. Some clever developers actually use these as bridges towards their flagship products asking for a paid license (their advertisement being their free products): that's what iObit does btw.
What it otherwise would be
The aggravation rises when users pay for a product that keeps acting as if they'd never paid anything, in regard to your advertisement activities/revenues: the common, justified and legitimate perception being that even though they've paid for a product, the company continues to consider them as their ow products for selling advertisements, as an asset for consolidating advertisement revenues or the object of whatever agreement iObit could come up with third parties! Your paying customers are people, as the word "customers" implies, not "assets" that can be sold out to advertising customers or third party partners.
Imagine if every business on the Web was acting like this : we'd go back something like 10-20 years ago when it became a real problem, and the main reason for installing spamware/malware/spyware/adware fighters 😉.
In short - it's not because something would allow you to make more money, that it's necessarily good: since you're dealing with human beings rather than mere "economical agents", there's a backdraft in the balance and to take into consideration. For instance, paying customers who stop to be paying customers (losing some grounds on your customer base), as the OP implies: it's a matter of mutual respect...
Simple Solutions
For paying customers, there could be two possibilities:
op-out from advertising e-mails;
when a customer leaves an advertised product like iTop VPN on the side (maybe they don't want a VPN or already have one), then iObit should STOP feeding them with such, permanently, unless the customer chooses to change that behavior from some option parameter.
Iobit makes great products that can speak for themselves, which is why I took the time to write this post, in all respect. Please, accept my best regards and have a good continuation.