From the question I understand that the company is small. Get a cloud-based invoicing system like FreshBooks - available from everywhere, very inexpensive, highly configurable and integrated. You can integrate it with accounting like Simply Accounting. You can also integrate it with Cloud-based Customer Relation Management system like BatchBook. The combination of these 3 services will give you full control over your customer data, cash flow and invoicing. There is a nice analytics built right in that helps making right business decisions. My advice would be to go to FreshBooks site, copy and paste their "reshBooks Makes Billing Painless" points to your presentation. Same with Batch Book, etc.