I've only ever received folded certificates in response to faxed requests for 3 year searches etc.
Up to & including yesterday, my online orders continue to come flat in a large envelope via TNT.
My understanding was that this was the driver to change to TNT's postal service - customers didn't want folded certificates & the Post Office wanted to charge too much for A4 envelopes.
I have NO IDEA why they fold the fax / letter requests because as you say, it is REALLY IRRITATING

SandraC