I am selling my co-ownership lot. The managing agents have invoiced me for the dated state 500€ including tax. Is this normal?

No. When a co-owner sells his lot within the co-ownership, the managing agents costs and fees relating to the services provided for the benefit of one of the co-owners are attributable to him.
However, the costs and fees relating to the establishment of the dated statement may not exceed €380 including tax (art. 10-1, b of law no. 65-557 of July 10, 1955, and art. 1 of decree 2020- 153 of Feb. 21, 2020).