Copying some keys at the store is more hassle than it needs to be. For small padlocks and other very simple locks, you can copy some emergency spare keys with the lid of a food can.
This video from YouTuber DaveHax shows how easy it is to copy almost any simple key at home. Take the original key and hold it above a lighter just long enough that it begins to char. Then press the dark part of the key against some clear tape. Once you have your key outline, place on the lid of an empty food can and cut out your copy. Keep in mind, however, that this copy will be very thin and should only be used in an emergency. When you do need to use it, hold as far down the key as you can, and turn slowly and carefully. The last thing you want is to break it off in your lock. This copying process is probably best for keys that go to very simple locks, like lightweight padlocks, cabinets, and diaries.
Emergency Spare Key [YouTube]