I had never done option A because I read reviews before that they (agent) charge higher than you picking it by yourself.

But, I had done option B. Yes this can be done by the day itself, but be prepared:
1. Your time going to KLIA, which will take about ~30-40mins (1 way) from Subang and depending where you stay
2. Petrol and toll fees
3. Purchase bill, list of items/receipts
4. Maybe prepare tons of cash, at least maybe RM500 - But recently I heard now customs accept card payment already, the last time I went there was year 2022-2023 and they said they only accept cash.
5. (optional) spare clothing - especially long pants + covered shoe, because you are entering government(?) building and the guard has right to refuse you to enter if you wear short pants, saw someone kena refused to enter before right in front of my eyes because he doesn't wear long pants, in the end he said have to batal balik to Damansara, lucky I have one spare long pants in the car and I sold the pants to him lol
You need to park here:

https://maps.app.goo.gl/d66sNYjjZn7RkH6X9


you should be seeing POS Office on the other side, you cannot "drive in" directly there but park at the outside parking lot (shown in red circle in the picture I posted above), try park your car near the overhead bridge for your convenience, then you get down of your car and take the overhead bridge, head to the gate and give your IC and the shipping code (for yours its ELxxxxxxMY)