I thought I shared my experience dealing with SIRIM for a product I ordered from Amazon (oculus go) and keep this thread updated.

Day 1: Got an email from courier service about item stuck at Kastam (6:30pm)
Day 2: Sent email to Sirim along with docs from courier company - kastam sheet, airway bill and Amazon invoice (8:00 am)
Day 2: Sirim replied with instructions and forms (10:30 am)
Day 2: Replied Sirim with the filled & signed forms, other docs, and MB2u payment slip (RM100) - make sure to use Reply All (2:30 pm)
Day 2: Sirim acknowledge receipt (2:45 pm)
Day 2: Received 2 emails from eComM - as per earlier post here - ignore (4:00 pm)
Day 3: Received the final approval email from Sirim and forward it to courier company (9:30 am)
Day 3: Received acknowledge email from courier company and will proceed with clearance
Day 4: Items received (10:30 am)

Some note:
1. Prob would've got the item earlier if courier company sent the first email earlier i.e. before the working hour ends.
2. Provided as much info about your item and make Sirim's job easier (and faster)
3. Any product that has any inkling of radio signal usage will most likely be held by kastam for Sirim approval (bluetooth, wifi, 3G, 4G, etc)