Hi Maria,
You can confirm if billing plan is enabled by checking if there is some value available for the field "Billing Plan Type" in the Item Category configuration and also for the SD Document type. You can check the same using the transactions VOV7 for Item Category and VOV8 for SD Document type.
If some value is available then please check if making this field blank will satisfy your requirement as Santosh has already suggested.
Regards,
Som