In AI, when making envelopes, where stamps are posted, what about the dotted line of that border?

You can use dashed lines in the stroke tool. You can use any color you want. Draw a rectangle. Do not stroke when filling, use red. Set any large edge as needed! Then select the dotted line in the stroke toolbar! The length and spacing of the dotted line can be adjusted by yourself! Just adjust it according to your own needs.