I would place a criteria in the query by using an If, Then statement. On the
Criteria line of the query in design view, right click and choose Build, the
Expression Builder will appear and then in the left lower box you want to
choose Functions, then Built In Functions. Search on the right for the If
statement. It will guide you on how to use the proper sintax. On that right
side you will be able to select your query and select the fields you are
wanting to compare. The logic would be something like this: If the mail
address does not equal (<>) a PO Box, then select the shipping address. I
don't know how familiar you are with building expressions but you will have
to give the expression a name and then create the formula.
I hope this makes sense.