Use one of these instead: I will contact you with the tracking number today. I will contact you with the tracking number by the end of the day.

By the end of the day or within one day! (Within one day means within 24 hours from now, by the end of the day means 今天之内，今天下班前）

If you mean by the end of today, it would be “I will contact you with the tracking number by the end of today.” If you are trying to give a time frame then you can say, “ I will contact you with the tracking number within ____ hours/days.” Instead of contact you can say provide. “I will provide you with your tracking details “