It all depends on how they want to use the letterhead/invoice. If they want to do on demand printing of their letterhead using a Word template then there are a lot of limitations based on what graphic formats Word will accept. How well color is handled by Word when using that format. How close to the edge of the paper their printers will print and how close a particular type of graphic will match the colors chosen when printed on a particular printer (if they even use color printers/copiers). This may mean you want to conform to what is possible in a Word environment or create two versions: One for pre-printed use, and one as a trimmed down Word Template. As for Invoices all depend on how they generate the invoice. If they are using an Excel Template then the limitations are pretty much the same as Word. If they use some other application to generate the invoice then you need to know the limitations of the program. It may be they only allow the insertion of a logo on the invoice. So a custom invoice may not even be a consideration.
I work as an in house designer. We have pre-printed color letterhead that is used by the executives, we have a close approximation version in Word that is color and another in B/W. Most people use the B/W Word version. As for invoices they are generated in a custom database program and it is a “logo only insert” design. We don’t use pre-printed Letter size envelopes anymore. We simply use a B/W Word template, cost and ease of use.
So I would say deliver it in the best way the company can use them.
*Note on Microsoft products. In my experience solid colors do best when importing graphics. MS likes it’s own vector formats (EMF and WMF). If you stick to black they look great. Imported pdf’s print terrible. I recall experimenting with EPS files and was disappointed. I think the attempt included color gradients so maybe solids would work. I don’t recall. When designing for Microsoft products you need access to the program to test output. Some things look terrible on screen but print fine. Note my experiences are based on using Office 2003 products. Your experiences may be different with different versions.