If your admin has decided to allocate a prepaid budget for you and your default payment method is Company invoice, you will be able to pay using this option during the checkout flow.
Just go through the checkout as you would normally do until you reach the Payment page. Here you will be able to see whether there are any cost centers enabled for your user groups with prepaid print credit already allocated. If the value of the order you are placing is below the residual prepaid credit for that cost center, this is marked in green (see below). You will be able to select that cost center and use the prepaid credit for your order.
If the value of the order you are placing is above the residual prepaid credit for that cost center, this is marked in red (see below). You won't be able to select that cost center, hence you might need to select a different cost center or payment method (e.g. Credit card or Payment approval) for that order.