Sorry for the delay in responding. We've had to poll the group to recall how this one was done. The specific wording that's in the listing harkens back to the early years of the club, when all endorsements were processed manually. Obviously there's no way to upload a brag file to the endorsement checker.
We believe that when the checker was written, it implemented a rule that looked for the ADIF tag TX_PWR, which is defined as "the logging station's power in watts (>0)." If the checker finds a QSO that has TX_PWR included with a value between 0 and 5, the checker "awards" the QRP bumper sticker. We just don't know if that portion of the checker is still working.
So give it a try and let us know how it works. If this turns out to be another situation where the checker is not working, we'll figure out another way.