You can check for discounted costumes, costume accessories, etc., just by choosing “Everything Else” in the dropdown for Step 1 and entering the “keyword” like “zombie mask” in Step 3. This will show you the product type with appropriate discounts!!
Remember STEP 1 MUST BE ENTERED, then in Step 3, enter a keyword and choose the level of discount. Number of discounted items will vary by chosen discount amount. Higher discounts = less products.
The Amazon Discount Checker can be used to find other types of products and is not limited solely to costumes, costume accessories, etc.