I think the system of measuring and weighing cards is putting a lot of people off of sending stuff as it is so ambigous. Even within Royal Mail people find it annoying to deal with. And some of the mail that hasn't arrived yet, might be held up in the surcharge system. I think the best advice for future card sending is, stick to well below the size that might incur a surcharge and that way you wont be landing people with a bill.