To add an extra charge to a job (if you want to apply a rush fee, for example), you have to edit the Billing panel in the details of the job, select the extra charge option and enter a percentage and a description of the extra charge.
In the interface of the documents you create, there is an "Extra charge" column. Please note that this column is always visible in the interface, but it won't appear in the PDF documents if you haven't defined an extra charge in the job.
If you send your documents to your clients in another language (and if you have created another document configuration for this - see this article: http://help.lsp.expert/documents/can-i-translate-my-documents-in-another-language), you can translate the "extra charge" title by going to Settings > Document and email configuration > click on the configuration concerned > click on the Translations tab of this configuration. There you can translate the "extra charge" title in the language concerned.
If you would like to name your extra charge differently (for example if your only extra charge is "Rush fee" and you would like the column in your documents to be named "Rush fee" instead of "Extra charge" + a "Rush fee" description), click on the configuration concerned under Settings > Document and email configuration and go to the Translations tab of this configuration. There you can "adapt" the name of your extra charge.