When does the "(Includes Donation of $nn.nn)" show in the Item column of the totals section in the Receipt?
This text will show only when there is an Amount AND there is a Donation Amount value on the Payment Txn. This would normally occur when a Donation is given with a Membership or Event Payment.
The text comes from a Custom Label called "Receipt_Label_Includes_Donation". The value can be changed using the Translation Workbench