It depend on what you post.... It is different if you post document or post a postage...

If you post document. you can buy the envelope for poslaju.
If you post a postage, you only can choose pos ekspress or parcel

Poslaju, only document allowed. No postage. People who received nonid sign for it (if i not mistaken)
POs ekspress, document or a postage, more expensive abit compare to poslaju. people received nid to sign ( if i not mistaken)
parcel(postage), registered mail ( document), most cheapest way, also the most expensive way. easy to lost item