So, as I understand, I have paid 0.002BTC for 20 transactions. The need time I sign a transaction from my multi sign Electrum wallet, I will just pay 0.0001 BTC/tx, right?
No.
You have prepaid the 2FA fee for TrustedCoin's service... This is an EXTRA fee that you have to pay for Electrum 2FA service on top of the normal transaction fees.
So, because you prepaid, your next 19 transactions will not require a 2FA fee, as you have 2FA "credits" (click the little blue shield in the bottom right corner).
However, ALL Bitcoin transactions still require transaction fees.