If you are opting in to the 8% Flat Income Tax Rate, Taxumo can file a 0 tax Quarterly Percentage Tax Form (2551Q) for you. Note that this does not have a notation that you're opting in.
You can then, via Taxumo, indicate your opt in for 8% in the Quarterly Income Tax Filing (1701Q) due on May.
This is as per advise of BIR Client Services and some RDO's we called - with the purpose of making sure you don't have any open cases.
You may want to check with your specific RDO if that's acceptable with them as it's apparent that the RDO's are implementing the registration updates indicated in RR 08-2018 differently.
Note that, we recommend that you submit a Change in Registration Information (Form 1905) to either end-date "Percentage Tax" OR otherwise indicate that you are availing of the the 8% Flat Income Tax Rate. The RDO's differ in how they implement this so best to go to the BIR directly. You may need to submit an LOI, you can make a copy of a template we created.
Regarding the above, we realize that not a lot of people have the time nor the inclination to go through this inevitably lengthy process. So we worked with a reliable expert who can do this on your behalf. If you're interested, just send us a message via chat or send us an email at firstname.lastname@example.org.