Use a copy of your 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, you can provide a signed and dated written statement that details the gift items being shipped, their values and involved parties. We also suggest using Bicycle Blue Book to find the value of your gift, then taking a screenshot to document it.