Hi there, hhaidaryco17-gma.

You can create an item for the discount or service fee to the transaction of your customers. Then, add it to an invoice as a line item.

Here's how to create a service item.

1. Go to Get paid & pay and select Products and services.
2. Click New, then select Service.
3. Enter the name of the fee.
4. From the Category ▼ drop-down, select the category that best describes the fee. 
5. Fill out the rest of the fields as needed.
6. When you're done, select Save and close.
7. Once done, add it to the invoice.

If you've already sent the invoice, here's how to create a transaction for the fee: Add service fees manually to invoices.

For the discount, you can browse this article: Learn how to discount invoices and sales receipts. It helps you to set discounts with fixed prices.

Now you're ready to send invoices and record their payments.

Get back to me by commenting below if you have additional questions about the Price Rules feature. I'll be around whenever you need anything else.