I didn't realize the supermarket is so noisy!
I have tried and ultimately discarded a number of these sound amplifiers. They all suffered from the same defect--loud screeching feedback, which made it impossible to use comfortably.  They also fit poorly and tended to fall out.  These Banglijian sound amplifiers have none of these defects. They fit comfortably and do not result in feedback. The ability to set sound levels consistent with the degree of noise in the environment is a very useful feature. The volume range is well suited for my needs. As an added bonus they are easy to remove when my neighbor wants to discuss politics.
Rating: 4 stars