It is possible to regenerate orders that look perfectly OK in a preview, but the PDF has issues due to wrong fonts, CMYK/RGB, bleed and other printing intricacies you missed during the design.

Identify and fix the issues

If an order is placed, but the PDF doesn’t look 100% right to you do to a fault in the template or missing fonts you need to identify what went wrong, fix the issues and re-upload the template first. Test the template placing a test order

Regenerate output files

User input is stored in the database as part of every order. ZetaPrints web-to-print can re-generate the output files using user input and the current template files and fonts. The preview image will not change.
You do not need to contact the customer to repeat the order - simply go to the order details and click on Generate files again link. No emails will be dispatched and no fee applied.