You can take the letter (not your old disc) to apost office and do it that way yeah. 
You can tax before it runs out, up to a month in advance, but must not display the new disc until the old one runs out.
If you tax online you should get 5 days grace where you don't have to display the disc due to it being in the post, but do it early enough and it wont be a problem anyway.
You can tax before it runs out, up to a month in advance, but must not display the new disc until the old one runs out.
If you tax online you should get 5 days grace where you don't have to display the disc due to it being in the post, but do it early enough and it wont be a problem anyway.