In a Pay As You Go plan, you are paying a flat rate nominal fee (a few cents only) per order submitted in the system. However, If the number of orders are too low , you will still be invoiced for a minimum bill to cover the account maintenance costs (severs, bandwidth, security) .
NOTE: Pay As You Go counts the submitted orders in the system (either in TSO or QR code ordering, subject to your plan). It will include also the cancelled orders even if they are cancelled after submitting. ( Cancelled orders count and you are billed for them )