When you rent or purchase select items through amazon instant video they will give you $ 1 MP3 credit. I do not know which movies or videos qualify but I purchased the new Night At The Museum and received the credit.

Here’s the email from Amazon,
"Additional information on this offer can be found here.

As a customer who has purchased or rented a qualifying product from Amazon Instant Video, you have earned a $ 1 credit valid towards Amazon Digital Music albums or single songs. Limit one promotional credit per customer; if you have already redeemed this offer you will not receive a second credit. To redeem visit http://www.amazon.com/mp3 on your web browser, iPhone, or iPod touch, open the music tab on your Kindle Fire, or open the Amazon Music app for Android and begin shopping. Your credit will be automatically applied to your purchase(s) until you have used the full $ 1 value or it is expired. For redemption instructions and additional information, click here http://www.amazon.com/mp3redeem. Amazon Digital Music is available to customers located in and with billing addresses in the United States. Taxes apply in some states. Your credit expires at 11:59 pm PST on January 31. 2016.

The promotional credit must be used by January 31, 2016. This offer is subject to Terms and Conditions.

Thanks again for shopping with us."

The information on how to qualify is not very clear,
"To qualify:
Add all products described in the promotional offer message to your Shopping Cart one of two ways:
a. Via the Add both to Cart button in the promotion description, OR
b. Via the Add to Shopping Cart button on each respective product information page.
The benefit for this promotion will not be reflected on the final order checkout page. The merchant will apply any benefit after your order has been processed.
If you remove any of the participating promotional items from your Shopping Cart or violate any of the terms and conditions listed below, your order will no longer qualify for the promotion.
If you return any of the items involved in the promotion, the merchant will remove the benefit.
Terms and Conditions:
Promotional offer valid for a limited time only. Items placed in Shopping Cart overnight may not be eligible for promotion at time of checkout.
Promotion applies only to qualifying items displaying the offer message on their product information pages. Items that do not display the offer message do not qualify, regardless of the nature of the item.
Applies only to products sold by the merchant indicated in the promotional offer message. Does not apply to items sold by other merchants.
Does not apply to any products purchased on any other website operated by the merchant.
Applies only when all qualifying and benefit products in the promotion are purchased in one order.
Applies only to complete orders shipping to a single address that meet all other promotional requirements.
Offer good while supplies last.
No substitutions or rain checks.
Offer must be redeemed through the Shopping Cart. Does not apply to orders placed with 1-Click.
Shipping and handling charges apply to all products.
Add-on Items require a minimum purchase. See amazon.com/addon for details.
Offer may not be combined with other offers.
Void where prohibited."

This is Limit One.

Here’s a link to the last thread,
http://slickdeals.net/f/7092122-free-1-amazon-mp3-credit?v=1

If anyone knows how to tell what qualifies please share. Also first post so please be nice and comment with any changes I should make!