I would suggest you to do this procedure yourself, in-person; instead of involving any agent, etc.
When you will get tentative date regarding receiving of Smart Card, then once again you will have to go to office to receive the card, or you can also give power of attorney to someone to receive card on your behalf. Regarding power of attorney, I am not sure that anyone is eligible or just blood relative. You can get exact details from the excise office, but again photocopy of your CNIC and that person's CNIC may be reqd. in this case.
In any of these both cases, you will have to present the original letter and submit on the counter (and may be, show original ID card too for verification), which contains tentative time for receiving of Smart Card. Do have its 1-2 photocopies with you, and try to never lose that letter.
I hope, this all information helps you. Thank you.