I am working for a company producing carton and consuming paper, working for sales department. I helped the company to buy paper and the paper company giving me commission, is it legal or it is bribe?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.

It is not permissible to accept this commission as it is considered within the meaning of gifts and bribery given to workers. If you were not working in sales department and helped in concluding the sale transaction, they would not offer you anything. The Prophet warned us against this as he said: "The gifts for workers are considered as taking money illegally." [Ahmad] Besides, it is reported that the Prophet appointed a man whose name was Ibn Al-Lutbiyyah to collect alms. When the latter returned he said: "This is for you, and this is for me." So the Prophet stood on the pulpit, and after praising and exalting Allaah, he said: "What is the matter with a worker whom I sent to collect alms and he returns saying "this is for you and this is for me", why shall he not stay in the house of his father or his mother to see whether or not anything would be gifted to him, I swear by Allaah in whose hand my soul is, no one takes anything except that on the Day of Resurrection he would be holding it on his neck (i.e. as a form of punishment)…." [Al-Bukhari and Muslim] Imaam An-Nawawi said: 'This is evidence that the gifts for workers are not permitted.' 

Allaah knows best.