Thickness tolerance: what a buyer should put in the PO

A '1.5 mm' sheet may legally be delivered at the lower end of the permitted tolerance. That is not a defect; it is what the standard allows. If the part is welded, formed or load-bearing, specify the acceptance range and the measurement method (point, position, instrument) in the purchase order. Otherwise the mill, the stockist and the buyer can all be right and still disagree.