How do I add or remove services to an invoice once its been paid in full?
Thanks for reaching out here. QuickBooks Online has the versatility you need in situations like this. As LeithG has mentioned, you can create a credit memo for your customer. I'll be happy to provide more information so you're on the right track with your QuickBooks goals!
Rather than adding or removing items from the initial invoice, you can record a retainer or deposit. After you create the deposit or retainer item, you can turn it into a customer credit. Here's a helpful guide for your reference. You'll be able to track these transactions on the Balance sheet report.
Please don't hesitate to reach back out with other questions. We'd love to help!