Zip code range lookup (I use sql server 2015)
In my Orders table Postal Code is a varchar.

I want to do a search like this:

SELECT distinct email
  FROM [dbo].[tb_Order] 
  where postalCode in (20004,20528,20260,20529,22060,22181,20708)


(with many more zip codes)

I do not want to have to convert the list of codes into ("20004","20528","20708").

What is the best way to handle this?
If you're actually using Sql Server 2016, you can probably use the new STRING_SPLIT command to create a 'table' to join against.  Just put your comma delimited integers in the STRING_SPLIT command.  Here is an example.

declare @tb_Order table (col1 int, postalcode varchar(5))
insert into @tb_Order values(1,'11111')
insert into @tb_Order values(2,'22222')
insert into @tb_Order values(3,'33333')
insert into @tb_Order values(4,'44444')
insert into @tb_Order values(5,'55555')
---------------------------------------
SELECT a.*
FROM @tb_Order a
JOIN STRING_SPLIT('11111,22222,33333,44444,55555',',')   
    ON convert(varchar(5),value) = a.postalcode;