Giant Eagle doubles coupons with a face value up to 99 cents. For instance, if the coupon has a value of 75 cents and the product has a price of $3.00, then Giant Eagle will double the 75 cent coupon and the coupon will have a value of $1.50. That would reduce the price of the item to $1.50. However, if the price of the item was $1 and you used a 75 cent coupon, the coupon would only increase in value to $1. Giant Eagle does not pay overages on doubled coupons.
Source: Giant Eagle Customer Service email response Feb. 2012
Get the detailed coupon policy from Giant's website here