"___er than usual" is used when something that happens often is above or below the normal level:
The Shake Shack was quieter than usual last night.
When something is already at an extreme level compared to other similar things, and then it becomes more extreme than usual, you use "even ___er than usual".
In the example above, the speaker's phone bill is already high compared to other people's phone bills, or compared to what the speaker thinks she should pay. But then last month's bill was even higher than her already-high normal bills.
"Even ___er" can be used for both high and low extremes:
Bali is even warmer than usual at this time of year.
My computer is even slower than usual today.