if you cross the 50K limit and up to 100K $ limit that means you're in Silver subscription package. For those months where you cross the 50K Limit and within the 100K limit you've to pay 79$ only for that month.

If you cross the 100K limit and up to 200K you'll have to pay the normal monthly subscription fee of 158$ for that month.

if in the next month you have transaction less than 50K per month you will not be charged any subscription fees as they are in LTD.