I have small web desing company and would like to have some income from domains I'm selling to my customers.
Is there some way to buy 20 blank domain, 20 credits for lower price and then register domains?

Yes, many of the registration companies allow for "credits" or pre-payment into a reseller account against future registrations.

you can either resell domains via any domain reseller program out there , or sell via referrals, although reselling costs nothing these days :)