Another common practice by some dishonest contractors is to replace a simple small part that many times is not necessary. The capacitor is one of those parts that is relatively inexpensive but when added to a service call the cost adds up fast. Many times these are contractors that instruct their technicians to replace a part, any part on every service call regardless if it is needed or not.
Every capacitor has an energy capacity rating and when the capacitor is greater than 10% low on capacity this is when it should be replaced to prevent the motor or compressor from working too hard to start and/or run. A capacitors capacity is measured in micro-farads and every capacitor has the proper reading usually listed right on it and as part of the part number in many cases.
All capacitors must be replaced with the exact same size or capacity as is being replaced, never allow a different size capacitor unless some form of documentation indicates the incorrect size was installed on a previous date.