Did you ensure you put 'Historic' in the taxation class?, is it an age related number plate?
I'd give them a call to see what has gone on, you won't be able to get a tax disc at the post office as the form for the number plate isn't sufficient proof the vehicle is yours.
Have you checked with the DVLA site that the number is registered and taxed?, maybe an oversight on their behalf, the tax disc is usually stapled to the receipt for £55.
http://www.taxdisc.direct.gov.uk/EvlPortalApp/application?origin=vnav_bar.jsp&event=bea.portal.framework.internal.refresh&pageid=Vehicle+Enquiry" onclick="window.open(this.href);return false;