Part month or pro rata charges are added to your account when you add a service part way through your billing period. It is basically a way of billing you from the date the service was added until the start of your usual billing period.
Your first invoice with bOnline will contain part-month charges for your package and any boosts/ calling features, as well as your first month’s advance subscription charges. This can lead to your first bill being a little higher than you may expect, but there are no hidden costs.