After more than an hour on the phone with customer service reps who couldn't help, I managed to find this post that contained a solution that worked for me: https://ttlc.intuit.com/community/tax-credits-deductions/discussion/form-1040-line-12b-programming-e....
After using the Delete a form tool to get rid of all the donations I'd entered, this time when I clicked Wrap up tax breaks it finally took me back to the screen I'd been trying to get back to this whole time: Charitable Cash Contributions under Cares Act. Once there, it was as simple as editing the field that still showed $290.
