Hey @bests4him,
Thanks for posting in the Community. I'd be happy to help you with credit memos in QuickBooks Online.
Just to confirm, were you trying to delete the credit memo after saving it? The More option will not appear until the transaction has been saved. I'm including a brief video to serve as a visual guide:

If the More option isn't appearing after saving, I recommend clearing the cache of your browser as a troubleshooting step. Cache is designed to speed up browser loading times but can interfere with processes if too much is collected and cause unexpected behavior. The following article contains additional insight, as well as steps for supported browsers: Delete or Disable Cache and Temporary Internet Files in Your Web Browser.
Since you don't have the option for checking apply credits, does the credit memo appear as Paid when you pull it up? If so, selecting the blue link below Paid, then the blue link for the date will pull up which invoice that it's applied to. I want to ensure that you're able to credit the customers for donating the old textbooks and I'm attaching a video to serve as a reference for this:

Additionally, the following article is a guide for applying credit memos: Apply a Credit Memo, Credit or Refund to a Customer.
I'm just a post away if you should have any questions. Have a lovely day.