You have up to 56 days to pay on your monthly invoice, so you have time to go through it in peace and quiet before you pay. If you pay the invoice in full (the balance of the period), there will be no interest or extra fees.
You can use up to two payment-free months per calendar year (not consecutively) if you have made at least three monthly payments before. To use a payment-free month, you simply do not make a payment that month. Interest and administration fee will then be added the next month.