Transparent window envelopes with patterned paper showing through. What about a twist: cut an unusual design in the junk mail return envelopes, line with pretty paper and affix address label - letter behind paper?
How to make a Tracy Tag or Window Envelope Pocket using junk return envelope mail that usually gets thrown out. Try cutting out the front envelope with window separately for easier applications and decorate in a variety of methods.
Square Window Envelope + Card: Great for sending invitations or photographs showing images/texts from the window on the envelope - these suckers would be custom tho / would need to find the most cost affordable way to produce these.