GARDEN CITY - Long Islanders hit the stores today to cash in on Christmas gift cards, return unwanted items and to scoop up lots of bargains that even beat out Black Friday deals. In what's been dubbed "Mega Monday," retail experts say the day after Christmas is the third-busiest shopping day of the year. Retailers are luring shoppers in with huge discounts to get rid of inventory that wasn't sold before the holidays. The National Retail Federation says stores have already seen a 3.8-percent boost in sales this holiday season. That combined with many bargains this week has retailers optimistic that this shopping season will prove better than expected. Shoppers are urged to dig up any gift receipts and check out store policies before heading to crowded shops for returns and exchanges this week.