Today’s invention is a modification to the envelope.
I’ve noticed that, when carrying one of these to the postbox in the rain, by the time I get there, the address is often smudged and indecipherable.
Instead, I propose an envelope with a closing flap which is longer than usual. When it is full, the flap can be bent over and sealed along a gummed strip (white), as usual. This then lies over the address, protecting it during its rainy transit.
At the postbox, the extra length of flap is ripped off using the serration indicated, exposing the address (and maintaining the sealed flap in place).