1. Go to Setup 
  2. From the left panel, select Customize 
  3. Select Tab Names and Labels
  4. Select Rename Tabs and Labels
  5. Click the Edit link for Opportunities
  6. Click through to Step 2 and locate Opportunity ID
  7. Change the label text to Donation ID and save

This will now allow you to have the Opportunity term used throughout LCRM without causing the error you received while trying to do your mail merge.