It seems to happen randomly as to when I get mail. I tried traveling, but that didn't work. But then two Sim days later it came. But it certainly isn't regular. And I'm positive I'm not getting a 1 to 1 ratio of elements to items.
Claymask suggested - see thread mentioned above - that travelling is no longer a good thing when you are waiting for a delivery and I agree. You were lucky to get mail delivered two days later - I wish for that to happen in my games, but it doesn't. At the other hand, I have an absolute 1 to 1 ratio of convertion into elements.
Again, in that other thread, I mentioned how I had a reliable succes rate with sending off items after completing the Space Rock collection. I can now confirm that there are a few more collections that work just the same and this should allow you to collect all elements. Try, and see if it work for you too?