All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
If those customers leave that small amount of money out of forgetfulness or by mistake, then it is a lost and found item, and it is permissible for whoever finds a lost and found item of so little value to benefit from it. Also, if they leave it because they do not need it because it is a very small amount, then it is permissible for you to take it and benefit from it. Besides, it is also permissible for you to give it in charity without the permission of the owner of the pharmacy.
Allaah Knows best.