[Tdol]

Firslty, don't use contractions like 'you're' in a letter.

I would staart like this:

Dear Mr X,
I am writing to complain about an MP3 player that I ordered from you.

Then describe the time you had to wait.
I think you wanted 1GB and got 512MB- the numbers seem wrong.

Then end the letter with the action you want them to take stated clearly:

I would, therefore, be grateful if you could send me the MP3 player that I originally ordered without delay as this order has already taken a considerable amount of time