This methods adds a tip to the original sale transaction.
To add a tip to a transaction simply call TipAdjustment with the PNRef parameter set to the PNRef of the transaction you wish to adjust, and the GratuityAmount to the amount you wish to add.
.NET Standard Notes
To call this method in an asynchronous manner call TipAdjustmentAsync instead. When the method completes (or an error is encountered) the TipAdjustmentCompleted event will fire.