Rest assured, re-sending documents is absolutely free.
Your allowance is only deducted the first time you send a document, so if there is something that wasn't quite right the first time and needs changing, you can edit and resend a document at no extra cost.
To do this, locate the document you need to change by clicking the 'Documents' tab in the Sidebar, then click on the document you want to edit and you'll be taken to the document page where the tracking is located. From here you can click Edit Document, and make any necessary adjustments. After that you can use the Re-send document icon to send it off again - you won't be charged for doing so.