Wow - just done some quick sums, I can see why Sky arn't keen to swap cards out that often.
Assuming -
each replacement card costs 1p to build, and print the letter;
They've negotiated a special postage rate, say 15p per letter;
Thats £2,250,000
Then add on Development/Testing/Training and then ongoing support costs. Thats a bit of a dent in budgets when advertising revenues are down, channels churn I expect is increasing, and the number of people either cancelling or not paying their bills is on the increase.
Having said that, If we assume that the average monthly payment from subs to sky is £30, thats £270 million a month, £3 million isn't much of a drop in the ocean! ChaChing! That goes some way to explain their near £5bn turnover last year