True - although I looked at this one that we (eventually) received and it didn't. The thing is, though, even if they do put that on the back, I'll bet the vast majority of people sending cards never look at the back - a lot don't even remove the price ticket! I know they should, but people just don't.
The card manufacturers should bear it in mind when designing the cards. Fair enough if it's obviously a large one, but a few mm over the limit is just asking for trouble. Even if everybody who bought that card knew that it would cost a large letter rate to send, would they consider the extra cost worthwhile for an extra 5mm of margin?
Of course, the card manufacturers and purchasers are not blameless in this; but my point is that what was originally brought in with the justification of charging people extra for 'outsize' items, to account for the extra costs that RM incur in handling them, was surely never intended to cover what almost everybody would consider to be a very standard (even on the small side) card.
Considering, in 'the old days', you could just know that 'it's a card', slap a stamp on, post it and forget about it, I think it's a real backward step to have to think about checking for a small symbol on the back, or getting your ruler out if there is none, for every single standard card you send. In a world where fewer and fewer people are sending physical cards anymore, it's madness to make it harder, more confusing and less convenient for people to send them. Not to mention that a card received a week late, for which the recipient has had to go online and pay a penalty charge, is very much not in the spirit of what anybody would want when sending a greetings card and destroys all good wishes.
As the amount of cards and letters sent in the post plummets in the internet age, RM seem to have resigned themselves to mainly being a parcel delivery service... and yet, whilst Amazon (and even Evri) send you tracking/delivery updates and bring your parcel in their van promptly, RM bring you a card, lying that you were out and then give you more admin to actually get it.
You can't compete with a rival for long by deliberately offering a far worse, much less efficient and convenient service than they do! To be honest, if Amazon decided to start delivering post as well - which they could do at any time, I guess - that would be the end of RM in no time at all.