If you are in the middle of an order and need to set a custom rate, there's an easy way to do it after selecting the product. Follow these steps:
1. Select an available product to rent and tap the blue "Add to order" button in the product pane. If you're adjusting an order, simply click the line item for which you'd like to set a custom rate.
2. In the "Order Item" pane, you'll see the option to "Set custom rate".
3. In the pop-up box, add your custom rate. The product cost and order balance will adjust accordingly on the right side of the screen.