You can add in as much as you like. If it’s heavy, they have to pay extra postage. I have heard of people taping the return envelope to a box with a brick in it. That’s a lot of effort, but it requires very little effort to enclose a wad of extra junk mail.
If you want to repeat this exercise in order to cost them the most money on return postage, be careful not to send back anything with your name on it. If you want them to cease and desist with sending you junk mail, then including your name in a heavy return envelope will get you off their junk mail list much faster than politely asking them to take you off.