Plivo is a Pay-As-You-Go API platform. Plivo relies on its extensive carrier network to deliver messages successfully. In some cases, messages may be sent from Plivo but are not received at the end user's handset. Plivo charges for messages that are successfully 'Sent' out of the Plivo platform. 

Do I get charged for inbound messages when the message_url or message_method is not specified?

Yes, Inbound messages are free for Long codes but are chargeable on Toll-free and Short codes, even without the message_url or message_method configured. In this case, the inbound message state will be set as received in logs.