A LaTeX pkg that can add markups to make a letter become its own envelope -- would be nice!

This letterfu site has PDF templates for folding a letter into a self-contained envelope. Could be quite useful but not great that we need to hand-write the address info. Would be useful if a LaTeX letter pkg could do the complex fold lines and addressing for us.

Update

Anyone who is inspired by this project idea might also find these folding approaches interesting, apart from the link of the thread:

• 3 methods for Origami envelopes (perhaps only the 1st technique works on recangular/A4 paper)
• Irregular-but-symmetric hexagon result from A4 paper