Now where have I seen those shapes before..?
If only those symbols appeared somewhere else, too.
Using the large symbol on the front of the very first postcard you can assign a symbol to each of these shapes. Using the guide on the back of this postcard you can assign a letter to each of those symbols. Taken all together, you can extract the plaintext: "Anaconda."
Use all 3 parts together.
Now, how to possibly line up all these letters...
You'll need a map of some kind.
Using the line pattern on the front of the first card you can determine which letters you need and in what order. When extracted, you'll get the plaintext: "Send me the confirmation number."
What other card has already been tied to this one?
Have you solved the other stamps?
The numbers translate to the word "Tannin."