I am sending this message for everyone who may check in this topic and having similar issue.
I finally figured it out. In mine situation, the monthly bill I’ve got is for the API usage before the date that I purchased $100 topping up, which means there is a time gap. As long as this can be paid off, the topping up will start to cover the date from the purchase and onwards.
Hopefully my case can help all you who may come across the similar issue.