These are some very specific shapes.
In fact, if you put these all together they seem reminiscent of a different shape...
...back on the first postcard.
Final Solution: 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."
How am I supposed to line up all these letters?
If only you had a map of some kind...
If it helps, you've already connected this card to another card.
Final Solution: Using the lines from the first card with the shape on the front, you can trace a path that leads from letter to letter. When extracted, you'll get the plaintext: "Send me the confirmation number."
Have you solved the other stamps?
Final Solution: The numbers translate to the word "Tannin."