Posting something using Royal Mail?

I may sound stupid but I'm sending a small package (iPhone charger) to a friend of mine who lives in Manchester (I'm in London) I've put a 2nd class stamping on the package do I have to go in to a Royal Mail or can I just put it in a Royal Mail post box?

Post box if it fits

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I'm a Postman. A 2nd class stamp will probably not be enough to cover the postage cost of sending a charger - unless its less than 5mm thick.

You will have to go into a Post Office to make sure that your friend does not have to pay the missing postage and a £1 admin fee, and to stop the inevitable delay caused by the underpayment.