Selling Custom Decals are fun and lucrative...
however, once in a while, a customer says,

"Hey I ordered a decal that is supposed to say (whatever)
and you sent me a decal that says (whatever).

It is possible that the fulfillment team made an error, but most of the time,
the customer has forgotten what they ordered.

You should first, send an email thanking them for their order and
apologize that they experienced any problem with their order.

Then ask them to send you a picture of the decal they received so that 
you can confirm if there is an error or not.

After you get a reply and a picture of the decal...
then you can verifty they actually received exactly what they ordered.

Once you verify the product they received does NOT have an error,
you can reply back to them.

Include this video link in an email reply to them after you get a picture: https://www.screencast.com/t/1x696NHU

Take a screenshot of their order details from your amazon account and attach it to your reply.
This will show them that they did in fact recieved what they ordered and then
you can send them a link to watch a video that will show them how they 
can look in their own amazon account and verify that they did receive what they ordered.

IMPORTANT: Amazon does have a rule that says, Seller are NOT required to give refunds on Customized products.

You can see the info at this link: https://sellercentral.amazon.com/gp/help/G201757520

Scroll to the bottom and you can see what Amazon says about the return policy.

Lastly, if their decal does have an error made by the fuifillment team, no worries,
apologize to the customer, contact the helpdesk and the fulfillment team will re-create and
send out another decal at NO CHARGE to you.