Associating a rate with a customer is the wrong way to think about things. There is no linkage between customers and line items on a sales invoice. The reason is that you might sell different services to the same customer at different rates, either yourself or with employees.
You can do what you describe with Non-inventory Items under Settings. Read the Guide:
https://forum.manager.io/t/non-inventory-items/7344
Be sure to check the box saying the item can be sold. An entry might end up looking like this:
If you had multiple rates for that customer, you would need multiple non-inventory items. The advantage of doing things this way is that–with a different item name, of course–you can use the same non-inventory item for any customer. For example, you might have one for high-level consulting work, another for graphic support to charge for report generation, and a third for on-site training sessions.
Non-inventory items cannot be used for billable time. But they are really useful for fixed-rate sales. If you use them, you need to make sure you have an income account besides Billable time - invoiced, because that is where Manager posts entries from the Billable Time module.