This behavior is observed when an advanced event option has been enabled that controls the behavior of receipting on additional gifts at the time of registration. To disable this setting, follow these steps:
  1. Find and edit the desired event.
  2. Go to step 3. Select Event Options.
  3. From related actions, select Edit Advanced Options.
  4. Find and unmark the check box "Yes, disable PDF receipts for participants who make an extra gift or donation to themselves at the time of the donation" which is near the end of the page.
  5. Click the Save or Next button to save.
When this setting is enabled, the system assumes you are also going to be sending PDF receipts through Receipt Manager and thus it omits the tax deductible portion from the summary.