The invoice is included and it says "Credit $X.00"

This means that your account with us has been credited for that item.  We realize shipping speed is important.  We have 226,000 items in stock.  Some with very similar or even the same names.  We have items that sometimes are not found when we pick the order.  Instead of holding you up, we ship it out and credit you.  Then when you come back next time you can order the same item or something else.  Sometimes an item is misplaced and its located later. 

How to use a credit.

The only way to use a credit is to log into the site.  You can't expect Amazon to know who you are unless you log in.  Its the same thing.

1. Mobile:   click on the person icon.   
2. Desktop Click on "Log in"
3. Enter your email address
4. Check your email for the code
5. enter the code
6. Once you a logged in, just add items to your cart.
7. When you go to the payment page you will see your credit.