Tax payment, net or gross?

[g_urgeghe]

I've been struggling for weeks trying to understand. I have a VAT number and I am a freelance based in Italy, I have asked many accountants and freelance colleagues but everyone has a different opinion on the matter. Should I consider the net or the gross amount? example: customer pays 100 $ to upwork and I get 90 $ for my service (10 $ upwork fee) do I have to invoice for 90 $ or for 100 $?