I heard about a company that sells numismatic gold (of limited edition) through Internet and delivers it by mail. This company awards you 10-percentage commission for each customer you convince to buy a product from this company. I want to know if this concept is Halal?

Praise be to Allah, the Lord of the World; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

The main condition for selling gold against silver or currency is that the deal should be hand-to-hand for the Hadith that the Prophet (Sallallahu Alaihi wa Sallam) said: "Gold for gold and silver for silver should be exchanged same amount by the same amount and hand-to-hand. But if these kinds (mentioned in Hadith) are different, trade as you wish but from hand- to- hand". [Reported by Imam Muslim]
Since this company sends the gold by post, so the condition of dealing hand-to-hand is not met. Thus, neither purchasing gold from this company is permissible nor informing others about this company because it is a kind of cooperation in committing Haram.
Allah knows best.