Used stamps exchanges.

This is a suggestion for the site. Can there be a shipment information field added. So, once the stamps are shipped, the sender can put the site URL of the post office/ courier and consignment number for tracking the letter. This will also help receive r to track and know that a letter would arriving for him/her.

One important aspect of this clubs is the surprise element. You don't know who or from where a letter is coming. It's a surprise in your mail box.