You have a strange postal service. Where I live, the norm is that you put your letter in an envelope and it would be illegal for the postal service to open the envelope. If you pay the postage, your letter will be delivered, no matter what the content, and without any changes to the content.
There are a few exceptions, nevertheless, the postage itself is kept unopened. They're usually detected by specialized test equipment with very low false positive rates.
1) Explosives and biological agents. Likely detected by sniffers.
2) Radiological materials.
3) Heavy packages or letters may be opened and inspected, especially for customs purposes. Depends on the postal service.
4) Potentially xray may be applied.
None of this is really an obstacle to communication.
By the way, phone service is not censored or modified in most countries either. Including cellphones.
(1) and (2) do not really apply to letters (i.e., communication), (3) only applies when crossing borders, and then it's not the postal/delivery service that is opening the package, but customs, and (4) at least over here would be illegal, too, if it can be used to read the mail. Both opening/reading the mail that you are supposed to deliver as well as listening to phone calls going through your network is a criminal offence.
good luck with that one. check out the "postal wiretap"; don't think the government decided to build a "national" communications network for the fuck of it.
