You can try doing it online. Your location is irrelevant. Otherwise do it by conventional mail, but you must post it 15 days before it's due. Otherwise get your hotel to file a TM30, and then go to your local immigration office and file the 90-day
It's very possible! On a 12-month extension you need to report three times, but you can only be fined once. I know people who do it. They reckon it's cheaper than the taxi/train fares to immigration!
It's no big deal for immigration. They're more concerned with overstays. The 47 and 30 are just administrative processes which get them extra dosh in fines