How can I best apologize and make it up to a dear friend?

I was supposed to help a very dear friend move into her new apartment today. She’s moving just two blocks from me and I’ve been helping her while she looked for a place. We also work together. She recently helped me move into my new apartment. Well, my alarm didn’t go off this morning and I slept in way too late. She called me once to check in, saying “no rush”, but then I actually fell back asleep. I feel like a complete and utter horrible friend. I took dinner and flowers over to her apartment tonight and helped her move some last minute things. I apologized and I cried because I feel so bad. She said it’s not worth beating myself up over, but I know she has to be upset. Is there something I can do or say that might make her understand how badly I feel or otherwise make it up to her? She helped me tremendously when I moved recently and now I feel like I’ve lost the opportunity to repay the favor. Any suggestions you can provide are greatly appreciated.