If you only have a few customers (you probably wouldn't want it to remain that way), you could use the approach demonstrated in:
https://www.msofficeforums.com/word-...html#post46903
For a large list, you could use the approach demonstrated in:
https://www.msofficeforums.com/word-...html#post46287
Another possibility is to use a mailmerge - for which you can select an individual customer or multiple customers from a data source such as an Excel file and generate multiple such letters at a time.