How can I assign a fixed SKU to 'Tips' in my store checkout?

[tonsvalleyshop]

Hi! 

I have setup a tipping option in my store checkout for buyers to donate to a charity we support. 

How do I set one fixed SKU for 'Tips'. Currently Shopify assigns a random SKU to Tips all the time, and its leading to chaos in my billing and shipping.

Kindly help.

[Mayhem]

Have you found a solution to this? We are also looking for a way to add a set sku for tips. Thank you.