It's the later bubbles that are a problem, in the last couple of years I've had a total of 3 Jedi cards arrive through the post with cracked bubbles. One of which was graded and two were not. Never had a Kenner Star Wars or Empire card arrive damaged and this to me says a lot about the later Jedi, POTF and obviously Trilogo cards which have always been easily damaged in the post in my limited experience.
To be honest if your spending hundreds of £'s on vintage MOCs and your worried about them getting damaged in the post only buy ungraded ones which are fully insured. If your buying a lot you have to except some will get damaged it's a fact, these toys are 30 - 40 years old and some have been sent all over the world dozens of times.