USPS stands for United States Postal Service. (regular mail). UPS stands for United Parcel Service. This is a courier service and offers many different levels of service with a wide range of delivery times and prices.
Fluid in the lungs is a broad term to describe two possible conditions that may give characteristic symptoms, like a bubbling sound in the lungs (rales) when breathing. The fluid accumulation may be inside the lung (pulmonary edema) or outside the lung (pleural effusion), in the space between the lung and chest wall.
Any abnormal density will show up in the otherwise air filled lungs. Usually the infiltrate will mean pneumonia, or some sort of infection with edema/swelling that is in the lung. You could also have a cancer for instance that would cause pleural/lung fluid build up that would be considered an infiltrate.