Write your message on the back of the jigsaw card, break the pieces apart, put them in the envelope and send it away as a regular letter (weight under 20 grams). The receiver must put the pieces together before he/she can read the message.
“Many thanks for your email and going to the trouble of seeing if you are able to help with the jigsaw I wish to try and purchase. I appreciated it very much.” Alison, Invercargill