"Overdelivery" is when your Hotel Ads continue to serve after your daily budget is exhausted. This overage is typically due to ads having already entered the auction process when you reach your daily budget. Overdelivery amounts tend to be small, and will be refunded to your account as long as your budget is at least $100 USD.
For example, if your daily budget is $90 and you actually spend $95, you won't receive a refund since your total spend is less than the $100 threshold.
However, if your daily budget is $95 and your actual spend is $105, you will receive a $5 for the overdelivery amount above $100. Similarly, If your budget is $100 and your spend is $110, you will be refunded the entire $10 for the overdelivery amount.