It's not that there's no extra cost to yourself, it's more that you are missing out - you could get the very same item through your own affiliate scheme, thus contributing to your own bottom line (read - paying less for the item).
"you could get the very same item through your own affiliate scheme"
FYI. The Amazon affiliate program does not allow you to purchase items via your own referrals. To do so is a contract violation and they will terminate your affiliate account.
