Priced per unit with quantity breaks. Vendors quote steps at 1 / 5 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.
The same product family is tracked in 3mm stock, 24×30, 3×5. Quotes are NOT interchangeable across variants, so match the spec before comparing.