Since I only have 3x WD Red 1TB and the DS413j is a 4-bay NAS, I went to Sim Lim Square to get another WD Red 1TB so as not to let 1 of the bay goes to waste. I didn’t realized that WD Red 1TB is a pretty old HDD and most of the stores carry 2TB onwards. Luckily Bizgram still has some WD Red 1TB left. So I bought it for S$120.
D-Link replied me after a week and a follow-up email from me. The normal repair price is US$61 (S$86) and after the repair is done, I will get an additional 3 months warranty.
However, out of good will, they are willing to give me 30% discount off the repair charges which is now US$42 (S$59) and an additional 3 months warranty, so that makes it 6 months warranty in total. They accept cash only and for some reason they do not quote in S$. Weird.