I presume you are registered for VAT?
You charge the net amount of the product cost and add VAT on to that.
If the cost of the goods is less than 2/3rds of the total invoice (goods plus labour) then, to use your example, you would charge €600 plus 13.5% VAT.
If the cost of the goods is more than 2/3rds of the total then you either charge the rate applicable to the goods on the whole lot, or you can split it out and charge 13.5% on the labour and 23% on the goods. But to do this you must state exactly what and how much is chargeable at each rate.