Mailbox

I made a mailbox (UK design) for Sophie to play "mail carrier" with. However, the letterboxes for the addressee have not been made yet (no time). I searched for a while for the tiny envelopes to be used as "letters" to be mailed out, and the Alphabet & Numbers rubber stamps. There are many variations to the game, limited only by your imagination.

Idea I
Stamp envelopes with capital letters and small letters (26 x 2).
Let Sophie to put them into the mailbox (slip them in).
Then she can take them out and sort them according to capital or small letters (2 piles).

Idea II
Using the same set of envelopes (capital and small letters), get Sophie to mail the "letters" and then retrieve them for delivery.
Have slots (boxes labelled with letters) and let her match the envelopes with the slots accordingly.

Idea III
Mail assorted letters and let Sophie "discover" them in the mailbox daily and deliver them to their slots accordingly.

Idea IV
Do the same for numbers, shapes, colours or anything at all!