What visa are you currently on?
>that 90 days report thing is completely unrelated to the topic of extending visas, right ?
Yes.
>And it means that every 90 days I have to do that, as long as I am in Thailand ?
To submit it in person, you have a window from 15 days before it's due to 7 days after the due date. To submit it on line, you have a window of 15 days before the due date up to the due date. For mail, contact your local immigration office.
>Can I do it online ? If so, where and what to know ?
use one of the following three applications:
Immigration web-portal [best option];
tm47.immigration.go.th
Android eServices application;
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iPhone eServices application;
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>what is the cost ?
Free. Not submitting the 90 day report will result in a fine of 2K THB the next time you attempt to acquire an extension of stay.
>I will reach the 90th day of stay on Wednesday 9th of March, meaning it has to be completed on Tuesday 8th max ?
It has to be completed by Wednesday 9th of March.
However, if this is your first extension of stay, you should have received a small slip in your passport that looks like that attached. What extension of stay did you acquire? If you got a 60 day extension of stay based on unable to travel due to COVID-19, did you ask the 90 day reporting desk what the policy is?