What is the meaning of "100 Transactions/Month." How is this limit calculated, and is there a possibility of increasing it if required?

What is the meaning of "100 Transactions/Month." How is this limit calculated, and is there a possibility of increasing it if required?

The 100 transactions refer to invoice transactions only, additional 100 invoice transactions can be purchased at $30 per month. You can increase this any time.

Ad-hoc invoices above the package limit is charged at $0.50 per invoice transaction.