Starbucks systems won't allow me to tip with a card

I go to two of the Starbucks in Johannesburg quite a lot and have noted that they don't have the option for customers to tip the staff if the customer is paying with a card. I've asked a few times and the staff have told me that they don't know why this option is not available.

I also use Seattle Coffee just as much as Starbucks (I love both for different reasons) and they have an option for customers to tip staff if the customer is paying by card. As part of the payment process, the customer can select either No Tip or Tip and then can enter the amount of the tip. As customers, we use this option as Seattle a lot

The fact that Starbucks, a world class company, doesn't have this option is very distasteful to me. Tips are an important part of hospitality staff income. I'm disappointed that the higher ups at Starbucks are allowing staff to miss out on this important source of income. Starbucks staff are very pleasant and obviously love their jobs. They contribute to the great ambiance that we love about Starbucks. We want to tip them for that. Update your systems so that we can tip them.

I know that updating technology costs money. But don't try to save a few bucks at the expense of your staff. We love your staff. Allow us to tip them.

In this modern era, we don't carry cash around much, and therefore are unable to tip them when we don't have cash with us