Good point. You'd have to take it into the Post Office to get it weighed, as with all these embellishments it would probably weigh extra, and postage isn't cheap. Most people get used to just sticking a 1st or 2nd class stamp on a birthday card, but craftwork bits and bobs will add to the weight (and surely padded envelopes needed so that it doesn't arrive looking like a crushed biscuit). Not being a craftsperson myself (apart from doing some sewing) I'll probably never have any experience of making my own cards, although I do admire some of what they do in these demos.
I put a small piece of bubble wrap over the front of the card. I only use an ordinary envelope, hand made if I need to, the card is made to fit the envelope. The cost of postage is based more on depth and large letter rate is normally ample for a greeting card. And when it's a special card I'm happy to pay what it takes to be able to post it. I also get asked to make cards for people so others must also be happy to pay the postage.
So would I. If I'd gone to the trouble of making a card with embellishments, I'd want to make sure it arrived looking good, so in my book you need more than just a larger-size but ordinary paper envelope.
Some of the demonstrators put so many layers on the front of their cards, it's a wonder they stand up. It's no wonder Sue Wilson only makes card fronts with the amount of layers and dies cuts that she uses.