No matter how large your business is, you still need to be on a personal level with your customers. Customers want to feel important and they need to know that you appreciate their business.
You should be sending out thank you letters and at the very least, thank you emails. If you don't have the customer's physical or email addresses, you have bigger problems to address. Don't even get us started on companies that don't track their customers.
If you want, you can make it even more personal and give them a thank you call. This will allow for the customer to give you instant feedback and for you to correct any concerns, if they exist