What kind of postarge stamps do you think the Doctor would use?
I am pretty sure he will not use these once that have a picture on it of the Tardis.
These are United States Postal Service approved stamps that are availible in all kind of values to fit your needs.
But more importantly of course is that they have a police box on it like the Tardis from Doctor Who.
Your mail will not reach the destination at Tardis speeds but at least you know with the right postage it will arrive.
Come check out these Doctor Who Tardis Postage Stamps.