AWS SNS text message spend limit issue

In my project, I am using AWS transaction text messages for New Zealand. It includes a monthly spending estimate of $100. cost per message is $0.12146. However, after sending nearly 400 text messages, AWS SMS has stopped working.

In one of the graphs, it shows I have reached 99.8 (assumed this is the average SMS rate for the month). According to my calculation, we only spend $48.584 (400*0.12146). Could someone please explain why AWS stopped sending messages? Please refer to the below images for more clarifications.

I think you need to open a support ticket and ask aws why is it so, if you have the option. Otherwise get the get the daily usage report which can give you the exact cost of each sms being sent. According to aws

The prices below are provided for guidance only, and change frequently. Once you’ve sent some messages, use the daily usage reports to get the exact pricing per message in USD