SharePoint offers a great feature that allows you to integrate your document itself with SharePoint metadata. You perform this by implementing Quick Parts in your document and point them at the fields you want to use.
All you need to do is add your template to the Content type you are using in the library. Once that is done, you can then edit the document within the content type. When you get to the section for your customer data, click on Insert -> Quick Parts. In the drop down is a section called Document Properties. You can select the column you are looking for and Word will link the section with the metadata. You don't even have to set the value in the document. If you set the value in the list, it will reflect in the document and vice versa.
Let me know if you need any assistance setting this up if I missed a step in my explanation.
Hope this helps!
EDIT: Please note: You can add your template to both a content type and the library itself. I prefer content types, unless you library doesn't have a custom content type. If you use custom content types, you can add the template to the CT by editing the content type and adding the template under advanced settings (steps 5 and 6 here).
To add it to a library (if using generic Document CT) you can follow this write-up.