why my project is "inactive "?

You do not need to pay, only if you wish the model gets back on the website for a next sale (reactivation).

CGt offers you a sales platform as a service, activating a model for sale cost a small fee depending on the price of your model. This small fee goo's from your CG credits every time a model gets sold and reactivated automatically.

However, when you run out of CG credits the automatic reactivation stops, so you need to buy/upgrade the CG credits to make automatic activation back in effect.

There are some ways to earn CG credits, for example when you upload a model you get some CG credits, there is also the referral system and active members earning many reputation points also get CG credits when reaching certain levels.

How do models get DE-activated? Or are they all deactivated by default and then activated - upon paying a fee - once the model is sold? Is this reactivation fee on top of the 27% commission fee? Or is it actually the commission fee under a swanky name? :-)

Models are active by default when uploaded.

When you uploaded your models you got some CG credits for it.
When models get sold the CG credits go down (because of the automatic reactivation)

Your models will stop automatically get reactivated when you run out of CG credits, automatic reactivation will no longer be in effect. So you just need to get some CG credits up your account.

Upload more models ore buy some CG credits.

There is a list of the tiers (can't seem to find it sorry), in tier 1 models from 10 to 30 dollars activation is 3 dollars, tier 2 models from 30 to 50 dollar is 5 dollars activation, tier 3 model from 50 to 100 dollars is 10 dollar activation and so on, (don't remember exactly!)

This information has to be somewhere on this website, can't seem to find it anymore.

Amway's, it's very cheap, you get average of 90% of you revenue, keep also in mind that the CG credits can be booked as an expense. (Ad least I do)

Ps. there are no additional fees to be paid, don't know anything about your 27% commission, as far as I know there is only the activation fee, at least this is how it works for me.