Use Only Adhesive Stamps

At least I think that's what the handstamp says on this late Yugoslav cover, addressed to a Serbian printing firm by sellotaping on the envelope an address cut from an advert.

My Serbian is shaky, so firstly can a native Serbian speaker confirm or correct my translation?

If I'm right can we then start frenzied speculation as to where it comes from:
A single collector fed up with getting nothing but computerised labels on his/her mail?
A massed Serbian Philatelists' campaign?
Or the stamp sales department of the Yugoslav GPO?

actually, HUPER is a magazine targeted at teens/young adults. kind of a cross between Rolling Stones and Teen Beat magazines. while browsing delCampe looking for interesting covers, I have seen literally hundreds of pieces addressed to HUPER. I am just curious who, on the inside, is letting all these covers out?

Thank you. You are quite correct. I should have said 'publishing' not 'printing'.
You see - not only is my Serbian shaky, my English is going downhill as well

I think that the sender affixed the stamp himself, and than he gave the letter to post clerk in the PO (he didn't put it in the public mailbox)... and the handstamp says exactly that "sender affixed the stamp himself".

probably some kind of internal notification in the postal service, because clerks didn't have a time to check are all the values actual in the moment on the bunch of letters (it was serbian infla period), so that all responsibility stays on sender (and eventually receiver)??

Ah. That makes MUCH more sense. Thank you very much for the correction.