Which code do I use when billing a laptop repair — HSN or SAC?

Both, on different lines: the parts you fit (a screen, battery or keyboard) are goods and take their HSN codes, while the repair labour is a service under SAC heading 9987, typically at 18% GST. Repair-shop billing software like BytePhase lets you set the right code once per inventory item and job-sheet line, so every invoice generated from a repair ticket carries the correct HSN and SAC entries automatically.