What does "Business Reply No." mean

[my2leftfeet]

I am sending a payment to ESB Networks in their pre-printed envelope.
The first line of the address is

Bus Reply No. 263

I'm not sure whether I have to put a stamp on the envelope or not.

Anyone know?!

 

[Spock]

Apparently not, from the An Post website:

Business Reply and Freepost - maximising your responses

• .....
• Customers do not have to pay postage.
• .....