I'm in this same tracker, I think.

I'm going to put fresh (return) stamps on the envelope, cover that area of the envelope with an index card, then take the envelope to the post office and have them put their stamp thingie on top of the index card. The recipient will remove the card, and my return postage will be revealed beneath.

Same treatment for the address area (my address underneath the one I'm sending to).

I hope someone will speak up if I've got this wrong.

I like the index card idea. Ah, well. I'll figure out something.

How is the aforementioned index card attached on top of the stamps? I can't visualize this working so well.

AB

The index card is covered with tape (such that it can be removed), then postage put on top of that. :)

How is the aforementioned index card attached on top of the stamps? I can't visualize this working so well.

Packing tape. I leave a small triangle of the index card untaped at one corner and marked with a small X. The recipient can grab that corner and yank off the index card and the tape to reveal whatever is beneath (stamps or return address).

The index cards are unlined and cut down a bit to fit the areas. I have an abundance of unlined index cards left over from a recent logbook-making frenzy.