Do you mean taking your biometrics at the Post Office, or picking up your biometric card at a Post Office?
If you mean taking biometrics for the visa application, I mailed off all my documents for the FLR(M), and the Home Office mailed me a letter to take to my nearest Post Office for biometrics. It had a barcode on the top and my application information. The woman who did mine said it was all hooked up to your application electronically so I didn't need to mail anything back. This was last November so something might have changed since then.