I work in a Post Office and heres a good tip.
Special Delivery Items go off weight (only) when it comes to cost and not size like with standard mail. So if you have to send anything small like keys, debit card, dvd, games etc via special delivery - then send them in a big box. That way somebody from your delivery office will fetch it round to you, much earlyer then the postman.
0-100 grams = £4.60
101-500 grams = £5.05
So a debit card in a shoebox size box will cost you the same then sticking it in a small envelope. £4.60
A video game with case/manual will weigh about 150-175grams. In a jiffy bag will cost you £5.05, but you've got over 300 grams unused excess to play with before it goes up to the next price band, so stick it in a big box and get it delivered earler.
I was speaking to our Postman who picks up mail from our Post Office once and he was the one who gave me this inside information. 
90% of the time, Special Delivery 1pm Items which are too big for the Postman to carry are delivered at the same time as the 9am specials.
But then again it all depends on your area and how many 9am specials get delivered to that area.
