For the sort specification, use:
sort=query({!field f=PostalCode v=12345})+desc,geodist()+asc
You can sort on fields, score, and function queries. Function queries opens up a lot of possibilities, and the "query()" function query opens up even more, as seen above. query() will return the score of the query within it, which in this case is a search for a field having a certain value. I chose to use the 'field' query parser but you just as well could do {!v="PostalCode:12345"}.
By the way, don't forget the other parameters for geodist(), like 'pt' and 'sfield'.