San Francisco was the exchange office. You do not show the back of the cover but my guess now is no return address. If there was a return address the letter would have been returned to the sender for the additional 3 cents US postage.
Such auxiliary marks are quite collectible and does add some minor value. As to being scarce, that depends on the measure. All properly paid letters have no need for such, just the short paid ones, then within that group via SF, such is not too scarce. At time such marking was applied and then the item returned for additional postage with the needed amount specified.
It is a nice item. Without addressing the value of the postage due letter, the marking would be a minimum additional $5 or more.
Here is the same type of communication but from the recipient of the short paid matter in Japan which received enough short paid matter to make up at their own expense an informational label. While containing the same information as your handstamp, this is a much more scare item but with lower interest as it is not applied by the USPOD.

