Must be shipped to a physical address. We will not ship to P.O. boxes. Enviro-Safe Dye Inject is a detection dye that works harmoniously with refrigerants to locate hard to find leaks in the a/c and refrigeration systems. A colored highly concentrated dye (fluorescent yellow) is utilized, and is the same color as the dye that auto-makers use in automobile a/c systems. We only ship to physical addresses.