Visitor. I had a slight error in a gerber file, one solder pad for a edge connector had no solder mask (to expose copper). JLCPCB noticed my error and paused production to ask if that is what I intended. I said "no" and offered to send a corrected gerber - they said I didn't have to, they corrected it for me. Amazing service for the price. My total order (multiple designs) was over $150 but that prototype PCB was only a 5 piece, $2 order.