On the main page of the Tappr App add a custom price by tapping in an amount (pictured below).
If you wish to add another custom amount, tap the + and include the additional amount (pictured below).
From there tap on the item amount located at the top right of the price. This will show you both custom amounts as line items (pictured below).
For more information on how to process a custom price via the Tappr App, please watch the below video.