This might be a bit late, but still, someone might be able to use it.
Here is an advice from a Russian forum, where sending passports per post had been forbidden up to 2012. The advice comes from a DHL employee - you can unfold your passport and put it in a possibly thick magazine and send the magazine as a document. Second, on its way back, indicate a corporate address (e.g. that of your employer), as there is less chance it will be checked by customs officers.
Here is an advice from a Russian forum, where sending passports per post had been forbidden up to 2012. The advice comes from a DHL employee - you can unfold your passport and put it in a possibly thick magazine and send the magazine as a document. Second, on its way back, indicate a corporate address (e.g. that of your employer), as there is less chance it will be checked by customs officers.