In a press release Wednesday, RadioShack Corp. announced that it has expanded its agreement with Comcast Corp., by adding more than 600 stores offering Comcast High-Speed Internet in the East, Midwest, South and Mid- Atlantic regions. Customers, upon buying Comcast service at a Radio Shack, can receive high-speed Internet for three months at an introductory price of $19.99. Buyers can also receive diecounted rates on modems and special rebates. Radio Shack has more than 7,200 stores nationwide. RadioShack already offers the service at more than 1,250 of its stores within Comcast Cable's service area.