U.S. Office Products Co., owner of Mail Boxes Etc., said it is selling most of its major assets and is filing for bankruptcy protection because of crushing debt and increased competition. The company will sell its North American units for $400 million, including reorganization costs, to Corporate Express Inc., a unit of Amsterdam-based Buhrmann, the world's largest distributor of office supplies. Office Products said Friday that it will sell Mail Boxes Etc. to United Parcel Service Inc. Washington-based U.S. Office Products, which bought 200 businesses in a bid to be a leading U.S. office supply company, listed $1.38 billion in assets and $1.35 billion in liabilities in its filing with the U.S. Bankruptcy Court in Wilmington, Del. Office Products shares fell 3 cents to close at 5 cents in over-the-counter trading.