I don't know if the reason for asking for the deposit is the same, but a TV repair shop owner that was a good friend of mine started asking for deposits for TV repair when years went by and folks didn't pick up the TV's. He had to stor them and finally get rid of them and it was a HUGE expense
to him with all the charges HE had to pay to get rid of the unwanted items. This may be the same for the sewing machine repair man....