Where can I set plans. Should i get an extension? What extension?As of now, you need to set Pricing plans with per email credit like as you said above :
1 sent email costs 0.001 $. customer payed 100 $ and he can send only 100 000 emails.
So create a price plan where sending limit 100k email and cost is $100.
If your client want more then you can create more plans like that and assign same way.