It is hard to get list of current services, and even harder to find out what does they mean (so You pick right name for constant in English). They have same codes for different services (depending on which parcel You send). Or even better (worse?), they have service A and service B and service C which is actually "A and B". Strange in situation, when You can assign as many services as You want.
I did my best and You can use constants from module [CzechPost::Services] (lib/czech_post_b2b_client/post_services.rb).