Unfortunately, you promoted at 100%, so you would owe 100% of the sale to eBay in just ad fee. Then you would also owe your normal final value fee, as well as lose the tax and whatever your cost is in shipping.
Sorry to see that you made this mistake as that is absolutely terrible and surely a very costly mistake unless the buyer cancels the sale and you get lucky that your mistake is refunded and canceled. Otherwise, you will need to take the loss as a very difficult learning experience.
Please make sure you read through the policies, agreements, and options for promoted listings or any of the other programs/options you may use on eBay.
Simply testing something on a live listing without understanding what you are agreeing to can be extremely costly. Luckily, this was not something overlooked that resulted in a policy violation or something that could cripple your account. Most monetary mistakes can be dealt with or absorbed (in most cases), but receiving a violation or suspension from overlooking something is not anything you want to chance occurring, being that it could result in losing your account.
As a possible solution or attempt at resolution, you could reach out to customer support / request a call back and then to the promoted listings department, and explain the situation. Maybe they will give you a courtesy to your account so that you do not have to take the extreme loss. Also, some sellers are getting a fee credit for Promoted Listings this month. Maybe you got lucky and your account is enrolled in the promotion so that you will be refunded the add fee.
Truly sorry that you are going through this, as it does seem like an honest mistake from misunderstanding the Promoted Listing policies.
