Here are some ideas for little extras to send out with orders to make your products more memorable for your customer and hopefully become repeat customers.
Everyone: Include a thank you note or card.
Candles: Include custom printed matchbooks. There are a number of companies online that print them but most sell by the case (2500 matchbooks) which may be more than you need or out of your price range, if you want a lower cost solution consider buying plain white matchbooks and rubber stamping your logo on. There are several companies online that make custom stamps, RubberStamps.net has pretty good prices.
Jewelry & Clothing: Print up a notecard or pamphlet that includes care instructions.
Kids products: How about including stickers related to the product. As an example, if you sell sock monkeys include some stickers of monkeys. Dollar stores are good sources of large quantities of sticker books. Just cut the sheets into strips of 3-4 small stickers or 1-2 larger ones.
Bath & Body: Include a small sample pack of your products in some scents that compliment the order.
Supplies: Include a few extra beads, buttons, findings, etc. as a bonus. Also a way to use up odds and ends.
As always including business cards and nice packaging regardless of what you sell is a big help.