You may need to add a custom amount on the fly from the point of sale. You can do this via the add custom amount option as per the below instructions.
From the point of sale click the action menu which will display several options.
Select the Add Custom Amount option
This will add the line item to the checkout window.
Click on this custom line to bring up its details.
From here you can modify the amount, description, and the drop down info. You can add to the drop down by simply entering text into it which will be available for use the next time you choose a custom amount.
You can set any tax or discount options to it.
Then click the check button to save your changes.
You can add more items to the transaction if needed then click the charge or refund button when ready to finalize the transaction.