I added 600B. I had like 15B removed because of an advanced credit I had used.
The plan renewed as normal but added another 20B credit to cover the difference.
So, it looks like I was likely being charged VAT all along, I just didn't know because it doesn't tell you the exact amount charged. I only knew this time because of the credit.
Either way, I still got the same plan at the same price. 550B (+VAT) for 6mbps unlimited. I've been using it for the past 10+ days and haven't noticed any throttling. I probably use 2 or 3GB of data per day.