The web design company you choose for your project should have relevant experience. If you would like them to design an e-commerce website, it is best to choose a web designer who has e-commerce websites in their portfolio.
Your web design company should be ready to submit a proposal for your website project along with a contract. Beware of companies who don't give you a formal contract.
The contract should outline exactly what the web designer will do and how long it will take the price, and what you have agreed to supply. A formal contract is the best way to avoid disagreements between the web designer and you.
You must know that after your website has been created, from time to time you will require updates. Maybe they will be small, maybe not. If the web design company you are choosing does not have an update package, you should inquire about the price of updates after the initial design.
You should take the price of updates into consideration before you begin the design process. Make sure the web design company or web designer you choose sets realistic goals for the timeline. Most web design companies have already built a pretty good reputation in the market.