To find out how many invitations each person has to take to the post office, we need to divide the total number of invitations by the number of people mailing them.
There are 2 people (Susan and Benjamin) and 3 members of the bridal party, making a total of:
\[ 2 + 3 = 5 \text{ people} \]
Now, we divide the total number of invitations (225) by the total number of people (5):
\[ \frac{225}{5} = 45 \]
Therefore, each person has to take 45 invitations to the post office.
The correct response is 45.