Unless you know who you're dealing with, get bids to compare prices and quality. Never hire a company that doesn't first provide a detailed inspection and an accurate measurement of your lawn. Low measurements are used by unprofessional operators to provide a cheap price. Such deliberately under-measured lawns will invariably receive substandard service and inadequate material. Although a written guarantee is only as good as the company's quality control program, it provides some assurance if you have a problem.
Most companies have a continuous service agreement.
When starting service you may be authorizing the company to provide service to your lawn from season to season, and year to year, until you notify them to stop. Be aware of what you are buying and read the correspondence your service provides. Either granular or liquid delivery systems will produce a very good lawn. Avoid companies that promote one method by discrediting the other. Most companies have an organic line of products and make them available as an alternative to, or in combination with, traditional lawn care methods.