I'm expanding on Reach4 comments.
Warranty on parts is 1 year, 5 years or 10 year on the compressor. Labor is usually one year. In 2017 my three year old air handler ECM motor went bad under warranty. Motor was covered by Carrier but it still cost me a service call and install of $350.
You need to read the labor warranty, it probably doesn't include diagnostics and service calls. Many companies offer annual service contracts. The main problem with labor warranty is it's the installation and service company providing the warranty and if they are in business for another 12 years?
In your situation you are charged for a service call to determine there is a leak but the process & procure fee is all BS. You are being charged for them to go the supplier to get the part but they first need a Return Authorization from Diakin. It all takes time but to charge extra for that stuff is for a fall guy. Don't pay it.
They didn't mention that you'll probably be charged for refrigerant. Reputable and goods tech do use a scale to measure the amount replaced. It is extremely expensive. You probably have R410a that is being phase out by 2023 for new units so it pushes the price up. A three ton unit may take 12 pounds. Ask this company what they charge. Don't be surprised it's $70 a pound.
There is a process to evacuate any refrigerant to inside the compressor unit but really no way to know what has been recovered. Sometimes the tech has to completely evacuate all refrigerant left in the system, such as a compressor replacement but it cannot be reused due to contamination and chemical separation. Never reused refrigerant. R410a has different chemicals that separate and it cannot be added as a gas but as a liquid. R410a is a zeotropic mixture, or non-azeotropic mixture, is a mixture with liquid components that have different boiling points. You'll see techs with the refrigerant tank upside down on a scale as its being added to the system. Good techs want to know how much is being added along with the gauges so not to overfill.