Actually, the difficulty of this is not nearly as hard as one might think. There is a solution from a company named Stone Edge Technologies. It is a system designed to actually work across both brick and mortar stores and e-commerce and other sales channels to manage your orders and inventory.
Order Management & Order Processing with Order Manager From Stone Edge
THe solutions starts at $1500 and runs to $6000 depending on the level you need. While it isn't dirt cheap, it is a solid solution to offer a store looking to mix in online sales as well. Obviously, cart pricing and POS equipment are seperate fees.
If you are running a brick and mortar and are looking to get online as well, this is a solid solution that allows you to manage everything in one place. Definately worth a look if your client is interested in a single environment.
It integrates with a lot of environments so the company isn't locked into one solution. If you client is serious about it, this is worth a look.