(I am not sure other countries have similar process) In Korea, all employees have to report the actual income tax in January. If the employee has earned income related to labor, no profit from stocks, no business, and so forth, the all process will be ended and the company settles the tax from February.
And the final modification of individual taxes is performed in May. All people who have income can correct tex. Many employees don’t need to correct. So the process is finished in January.
I reviewed the May’s year-end tex settlement. (I don’t know when I finish my life as employee. I have to correct in May, someday.) I can see how much I earned and how much I spent, how much the tex is imposed. But the topic of this post is not my correction.
It is very specific case of Korea, and my situation. So I don’t translate my whole content in Korean.
In summary, there are several web / mobile service to announce reduce individual tex. (= get a refund) Sometimes the service warns ‘it is last time to get refund’ or ‘your refundable tex is not calculated for several years’. So many people visit the service and get a tex refund service.
In my case, the service announce I can refund about 384 k KRW.

My real refunded tex are, (I insert my information myself and I calculate the situation to get most refund and..)

More than 1.20 M KRW. (more than 3 times the service noticed)
I applied several refund so the total amount is more than above photo. But, anyway, what I think is
The people who knows how much I texed in January and how to refund most is..
I, myself!
The services just perform automated calculation. In many cases, they can get best result. But it is not always. Moreover, it has risk to over-apply the refund. I have experience to over-apply (not using such service, I had mistake in January’s settlement.) and paid lots of additional tex. The tex refund should be applied carefully.
The tex refund service is trendy in Korea. So some conflict between tex refund service and tex accountant association. I don’t know how it going on. But I suggest ‘study tex and apply myself is more safe and pay no fee’.