Praise be to Allah, the Lord of the Worlds.
Based on the details of the above question, it is clear that a customer pays, in advance, the price of a commodity that has specific features and isn`t in the possession of the seller. In light of Islamic Jurisprudence, this is a Salam contract (Advanced payment for deferred delivery). Narrated Ibn `Abbas: The Prophet (PBUH) came to Medina and the people used to pay in advance the price of dates to be delivered within two or three years. He said (to them), "Whoever pays in advance the price of a thing to be delivered later should pay it for a specified measure at specified weight for a specified period." {Related by Bukhari & Muslim}.
Once the Salam contract is concluded, the price of the commodity gets into the possession of the seller. Therefore, it is imperative that he pays the Zakat of that amount, even if the commodity wasn`t delivered to the customer. This is because possession is what renders Zakat obligatory.
Sheikh Al-Islam Zakaria al-Ansari (May his soul rest in peace) said: "It is obligatory to give the Zakat of the capital in a Salam contract after one full lunar year elapses, even if the commodity wasn`t delivered to the buyer."{Asna al-Matalib, 1/358}. Also, check Ibn Qhodama`s book {Al-Moghni, 3/72}.
a In conclusion, the seller, in a Salam contract, must give the Zakat of the amounts accumulated from prices of sold commodities once they reach Nisaab(Minimum amount liable for Zakat) and one full lunar year elapses over possessing them. And Allah knows best.