Hi there, Upstart.

Let me show you how to apply credit memo to a specific invoice. Please be guided by the steps below

1. Select the Company and Transaction Center and click on Credit Memos.
2. Select a credit memo from the list on the left panel.
3. Make sure that it has the correct Customer or Customer job applied.
4. Click on the Use Credit to button drop-down.
5. Select Apply to invoice.
6. Choose which invoice you are going to apply your credit by ticking the box on the left side of the Date column.
7. Confirm the transaction by clicking Done.

An alternative way is to use the Receive Payments option. Here's how:

1. On the dashboard, click on the Receive Payments icon.
2. In the Customer Payment window, choose the customer on the Receive From box.
3. Tick the box on the specific invoice, then click the Discount & Credits button.
4. Check the credit that you are going to apply and click Done.
5. Hit Save and Close.

For the processing fees, you can create an item for that and include it to your invoice. Then proceed to apply the credit memo.

Feel free to respond if you need anything else. I'll be here to help. Take care.