A simple print with a pointed “spear” to get under the envelope flap and a sharp edge (not a razor blade) to slice open the top of the envelope.
The spear is surprisingly robust because it is printed with the filament going longitudinally.
I found that using a little up-and-down motion when cutting improved the cut.
Replacing the PLA cutting edge with a razor would improve the cut, but assembly risk would increase. Maybe a razor design will be provided with the next revision.
The author marked this model as their own original creation.