Priced per unit with quantity breaks. Vendors quote steps at 1 / 100 units, and the per-unit rate drops at each break.
Usually substrate or finishing, not the printing itself. Ask both vendors to quote the identical substrate and finish and the gap normally closes.
Yes for short runs, but it should be charged once per design, not per unit and not again on a reorder of the same file.
Related variants of this product are tracked as separate benchmark lines. Match the spec before comparing quotes.