I send anything I can in an envelope – stickers, washi samples, tea bags, postcards, flat magnets, bookmarks, PDT labels, paper cut outs or punches, other paper ephemera, unused stationery and little envelopes, decorative paper clips, post-it pads – you name it, if I can get it in an envelope without it being to bulky to turn into a “parcel”, I will send it. I’ve also been known to put smaller envelopes into a large envelope to get around the “parcel” issue (if its under 3/4 of an inch in a large envelope, its okay and still much cheaper than a parcel). Sometimes it does require a bit of extra postage for hand-canceling or non-machinable, but I’ve learned what is acceptable to send without too many added costs.
And I am okay with anything in the envelope, I appreciate the effort and gesture! Someone sent me a face mask or similar one time and it exploded in the envelope – the post office put it into a clear plastic bag for me and the letter was gunky and smeared, but I loved the sentiment (and the fact that the post office tried to save the bright blue sticky mess for me, haha).