Return count of Postal Code based unique value

Maybe it will help:
try
1. copy postal values into another column
2. select this range
3. from the ribbon: DATA >>> DATA TOOLS >>> Remove Duplicates
4. in the next column use formula:
=SUM(IF(FREQUENCY(IF($A$2:$A$7<>"",MATCH($A$2:$A$7,$A$2:$A$7,0)),ROW($A$2:$A$7)-ROW($A$2)+1)>0,--($B$2:$B$7=H2))) with CSE & drag down

OR

(with your example)
in D2: =IFERROR(INDEX($B$2:$B$7,MATCH(0,COUNTIF($D$1:D1,$B$2:$B$7),0)),"") array entered and drag down
in E2: =IF(D2="","",SUM(IF(FREQUENCY(IF($A$2:$A$7<>"",MATCH($A$2:$A$7,$A$2:$A$7,0)),ROW($A$2:$A$7)-ROW($A$2)+1)>0,--($B$2:$B$7=D2)))) array entered and drag down

CSE or array entered = ...confirmed by pressing CTRL+SHIFT+ENTER to activate the array, not just ENTER. You will know the array is active when you see curly braces { } appear around your formula. If you do not CTRL+SHIFT+ENTER you will get an error or a clearly incorrect answer.