An overage is when the product you have is a price that is lower than your coupon.
Regular price: $2.39
Sale price: 99 cents (and you even price matched!)
You have a coupon for: $1.50
You have an overage of 51 cents per toothpaste you purchase.
This means that the coupon can be accepted (some stores say "yes" to the overage), some say "no" to the overage, but will accept the coupon UP TO the value of the sale price (example: 99 cents will be written on the coupon to show that you got the toothpaste for 99 cents, it's still "free" to you, but you don't get the difference). Some stores won't accept the coupon at all.
The BEST store for accepting overages is Walmart.
The best time to have an overage, is when you can get enough of one coupon to get the product overage and use the overage towards your other cart purchases (items that are more difficult for finding coupons and discounts for- like fresh produce, meat, eggs, etc).
Check a "coupon policy" for each store in order to find the specific way that overages are dealt with.