If you're marking the item as shipped, surely you can take the extra 2 seconds to enter the tracking number?

What "legitimate" reason could you have for not uploading within 3 days?

If you are offering a tracked service, then you need to upload tracking, otherwise Ebay will want to hear from the buyer that the item has been received, or a certain amount of time has passed without a dispute. Simplest way to get your money is to upload tracking.

Ebay won't hold your funds if you're offering an untracked service.