Kettle or bamboo splint. You use the kettle to steam open the letter. You use the split bamboo splint to roll up the contents of the letter and then pull the rolled up contents through the gap above the place the glue starts. I suspect that pressure sensitive glue is making the kettle technique less popular. Not to worry; envelopes with pressure sensitive glue tend to have wide gaps.
If you mess up then you very carefully copy the writing on the old envelope onto a new envelope and recreate any postmarks (if required). Even if you do a terrible job it is unlikely that anyone will notice.
If you somehow make it really obvious that the contents have been tampered with then you simply keep the letter or make it look like the sorting machines destroyed it. Then the post office gets blamed for the wrong thing.
If you mess up then you very carefully copy the writing on the old envelope onto a new envelope and recreate any postmarks (if required). Even if you do a terrible job it is unlikely that anyone will notice.
If you somehow make it really obvious that the contents have been tampered with then you simply keep the letter or make it look like the sorting machines destroyed it. Then the post office gets blamed for the wrong thing.