If you or the people making cards with you, are handing their cards to people, then you are ok doing whatever you want. I have seen some great "bling" on cards at craft fairs and there are no warning signs telling people buying the cards that extra postage WILL be required. You might slip by but if you have spent all that time deciding what stamps to use, what papers to use and then making a card, you don't want it destroyed before it reaches the person you created it for!
What do I do? I use Close To My Heart glitter trim. I use Close To My Heart embossing powder. I use glitter glue and I do dry embossing. I use the twine but only have it single layer and not tie a bow with it. With ribbon, you can find a punch that looks like 2 slats and make a "fake" bow that will go through the mail. There are many ways to have the same affect. You can use smaller glue dots and still have a lift with your small pieces.
I get it. Everything is modernized and all the mail goes through machines today. It used to be you could say "hand stamp" and it didn't cost you any more but those days are over. Over are the days that local mail was done through local people. Rather than saying "Woe is me", look at ways that enhance the cards without making them too thick or large for the mail. It doesn't take long to think of things. It may you have to adjust your favorite patterns but that is OK. You can still do the other cards with thick foam, tied twine and big bling. Just tell people that these will need added postage. I have used them and will gladly pay the extra postage to have the cards going to their intended addresses. It isn't a big deal as long as you are prepared for it and the people making or buying the cards know that fact ahead of time. Be creative but think ahead on where the cards are going and how they are going to get there.