You don't even have to explain yourself. Just tell her that you're not available to pick up the cake with her.
One of the rules of no contact is to only communicate when it's absolultely necessary. You've had a lot of unnecessary communications with her that is hindering your progress in the healing process.
Check out these two stickies, they should help:
https://www.askmehelpdesk.com/relati...qs-332732.html
https://www.askmehelpdesk.com/relati...es-351302.html

