Thanks for reaching out about this! It's important to me that you're familiar with all of the options for QuickBooks Self-Employed and have the best fit for your business in terms of what QuickBooks products can do. I'll go over this to make sure you have the information you need.
It sounds to me as though you're hoping to invoice a customer, but you're running into the issue of there only being the following three options for the rate you'll enter: flat rate, by hour, and by item. I can absolutely see how being able to bill per KM rather than these three would be useful considering that's what your client is looking for.
In this case, a workaround could be to use the Description field to make note that the rate calculated is per KM traveled and use one of the available options. For instance, you could use the flat rate option and include all of the information about the KM traveled in the description or use something like the "by item" option and simply indicate in the description that the item/unit in question is a KM. In that instance, it might come out looking something like this.
I know that's not quite what you're looking for, so I recommend leaving feedback for our product development team. You can do so by going to the Assistant feature (close out any transactions to access it), then typing and entering "feedback" and following the prompts. We love hearing the ways we can enhance what QuickBooks can do for our customers, so this is a great way to have a hand in that and have your voice heard.
If you need an extra hand with any of this, don't be afraid to reach out to the QuickBooks Self-Employed team directly. This can also be done through the Assistant feature, and this time you'd type and enter "talk to human" before following the prompts that follow.
Wishing you the best! Have a great rest of your day.