How are you creating the invoice?
Is it another worksheet in the same workbook?
=text(sheet1!a1,"$#,##0.00")
or
="You owe me: " & text(sheet1!a1,"$#,##0.00")
& " by " & text(sheet1!a2,"mm/dd/yyyy")
Or are you using MSWord's mailmerge?
Debra Dalgleish posted this:
There's an article on the Microsoft web site that might help you:
Answer Box: Numbers don't merge right in Word
http://office.microsoft.com/en-ca/assistance/HA011164951033.aspx
And if you prefer the old Mail Merge helper, Word MVP Suzanne Barnhill
has instructions here:
http://sbarnhill.mvps.org/WordFAQs/CustomizingWord2002.htm
about half way down the page.