Can the body text of a confirmation email from a Soapbox Engage app be determined by responses to the fields in the form? For example, could I have separate confirmation letters for those who donate under $10,000, and those who donate over $10,000?
Variables from the form submission can be included in email confirmations for most Soapbox Engage apps. Beyond that, the text of a confirmation email cannot be customized based on responses to fields in the form from within Soapbox. You can, however, create customized Email Alerts in Salesforce which are triggered based on Workflow Rules you define. If you do so, it is recommended that you disable automated emails sent by the given Soapbox Engage form and create the desired Workflow Rule for Lead records created by Soapbox.
For more on creating Workflow Rules, check out Salesforce's documentation.