By Nick Carey and Nandita Bose ADDISON, Ill. (Reuters) – In an unassuming building in the Chicago suburb of Addison, United Parcel Service Inc engineers design custom boxes for retailers as part of a joint effort to tackle a vexing issue: the high cost of delivering e-commerce packages. Online retail sales are expected to reach $392.5 billion this year, according to research firm eMarketer, but with U.S. consumers now demanding free shipping on most packages, shippers and retailers are facing a squeeze.