It isn't that commercial activities are unwelcome or unethical, on amateur bands. Rather, it's that they are illegal. The relevant rules in the USA are in §97.1 Basis and purpose:
(a) Recognition and enhancement of the value of the amateur service to the public as a voluntary noncommercial communication service, particularly with respect to providing emergency communications
and §97.3 Definitions:
(4) Amateur service. A radiocommunication service for the purpose of self-training, intercommunication and technical investigations carried out by amateurs, that is, duly authorized persons interested in radio technique solely with a personal aim and without pecuniary interest.
and §97.113 Prohibited transmissions:
(3) Communications in which the station licensee or control operator has a pecuniary interest, including communications on behalf of an employer, with the following exceptions:
While I'm not sure of the relevant legislation in your native Serbia, it's a fair bet that it's similar.
In any case, these regulations are about the communications you make on the air. The FCC has no authority over what you put in the mail. It's a fair extrapolation of the intent expressed in §97.1 and elsewhere that the FCC's concern is commercial interests using the amateur service as a free alternative to commercial spectrum which would otherwise require an expensive license.
There are many activities that do have some pecuniary interest, related to amateur radio, but not taking place on the air. For example, hamfests and magazines. Would anyone argue that these are unethical? I doubt it.
As such, I don't see any argument for making a paid endorsement on a QSL card any different than any other paid endorsement. Indeed, that can be an ethical minefield, but whether it's on your QSL card or your blog is largely irrelevant.