Well Andrea, I like to keep all conversations, questions and answers pertaining to one of my plugins in one single place. I find it more convenient for users also. And I like it for this place to be my own support and development blog, where most users of this plugin already posted useful advices or fixes.
As for billing support, I do provide personalized support to paying customers also. This is very clearly explained in the plugin help pages and on my site.
However, development of this specific plugin is funded by customers that agree for it to remain freely available to all.
I always prefer willing users to volunteer funding a new feature that will then be made freely available to all. I would rather suggest customers to sponsor an improvement to a freely available plugin, or make a donation to keep it free for all, rather than switch to the alternative commercial solution that you advertise.
I guess it's just a slightly different approach to the same business. Although I do respect other models and the quality of alternative software available elsewhere, I would just prefer to keep commercial offers clearly distinct from a forum thread dedicated to support on a completely free piece of software.