Use a copy of your original sales receipt (such as a bike shop receipt, screenshot of eBay auction closing or PayPal transaction) that lists the item(s) and displays their purchase price(s).
If you don't have this, provide a signed and dated written statement that details the gift items being shipped, their values and the involved parties. We suggest using Bicycle Blue Book to find and screenshot the value of your gift.
To see the documentation needed for this kind of shipment, please see the article 'What documents do I need to provide when shipping a gift internationally?'