Performed single system cooling maintenance. I checked amps, filter, electrical, drains, cleanliness, refrigerant temperature, and temperature differential. The system is performing inside of manufacture specifications. I noticed that the outdoor coil is oxidizing. To improve efficiency and temperature transfer, I recommend a chemical cleaning to remove oxidation. I provided price for a bottle of total-c and spoke about the benefit. Customer decided not to perform the biocide treatment and went with a general no rinse cleaning. I performed the cleaning today. I collected for the renewal and the cleaning.