I'm thinking to send a box of item to my customer. the box size is around 50cm x 50cm x 50cm.
and i found out that poslaju is so expensive since they use volumetric calculation.
i would need to look for another post service.
can somebody tell me which post service is fastest and cheaper?

To be honest, there are no best courier service out there. There are pros and cons for each one of them. I've tried SkyNet before and they followed the same procedures as you mentioned; I've to fork out extra.

Even private companies can be failures. I've a bad encounter with GDex and Kangaroo before.

Use nationwide..

Where you want to send? Maybe people can give recommendation base on the area you want to send. For me I would just use poslaju.

Poslaju is the best so far. But too expensive to send heavy and bulky parcels.
For bulky items, I'll just use SKynet. No problem with their service so far.
If you have a corporate account with them, they'll give u a zone 2 list which is those places that needs more effort to reach by skynet. Try to avoid shipping to those places, or else, the parcel will be delayed and being left somewhere in Skynet.