I think a markup on parts at double your cost is customary with most technicians. As far a keytops, I do a lot of keytop replacements and I suggest $100 over their cost as a figure to charge the customer. This covers the second trip to replace the keys, and possibly some leveling and minor regulation. A lot depends on your area, the customer and the piano. Mike Kurta, Auburn NY Home of the Wegman piano
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