I do understand why you are annoyed Oskar, but really the problem is with 1and1 for not offering you the choice.
Sergey is trying to explain that it isn't just a simple key upgrade that might enable or expand something that's already installed. There are some fundamental differences between this and, say, a key enabling one of the anti-virus offerings for more accounts.
I don't know for sure, but I suspect one of the biggest issues preventing PowerPack being offered as an add-on might be to do with support. If you could buy the PowerPack as an upgrade to a key provided by someone else, who would provide support if something went wrong? You might say that if the problem was with an element of the PowerPack it should be Parallels, and if it was with something else, it should be 1and1. But it isn't as simple as that, as the failure with one may be the result of a failure with the other. And what if you make a mistake during installation, and kill Plesk in the process. Should Parallels provide support in that case? Yes, maybe, but what if the installation problem was caused by the particular way Plesk was installed by 1and1 in the first place? Or some sort of unusual security or setup decision used by 1and1? Is it still Parallels who should provide the support?
But again I emphasise that I understand why you are annoyed. I am sure others are in the same situation as you are. Maybe some pressure could be applied on 1and1 to offer the upgrade? Or maybe Parallels could consider creating a special class of key with certain special conditions imposed, such as the customer having to pay for per-incident support is something went wrong panel-wide? Hmnm... maybe not-- too many things to go wrong or misunderstand. But even so, maybe this key upgrade issue could be something Parallels might concider for the future?