I like to add some value to my knives by making a Certificate of Authenticity and a Datasheet for each knife I make. These are inexpensive little touches that go a long ways for the price.I have made generic documents available for other knifemakers to use. Again, free as in beer.
The Datasheet is a Word document that contains a table which details things such as date of manufacture, serial number, steel type, handle data etc.
The Certificate of Authenticity is a Word document that can be filled out and printed on some fine stationery. You can get a corporate seal made up for around $50 and some gold foil seals. This is a great finishing touch for your handmade product.
One of these little desk seals with your name on it make for a great looking certificate.
Business cards are awesome to send along with your creations. You can add photos, text, your website address, email or whatever you like. I had mine printed at Vista Print for a ridiculously low price. By the way, they make bumper stickers too!
Other items to include with your knives might be a Care & Maintenance card listing some tips for keeping a knife in top shape or perhaps a basic sharpening guide with recommended practices.
All in all the package that you send is more than a knife. Often these finishing touches bring back the customers and their friends as well.