Oh gawd, I hate these things.
Not you, the conundrum of it.
Okay, you called her ahead of time to apologize to her. Sorry, you texted her. You also called her 3 days later, to reiterate why you could not make it. You tried twice and apologized.
You did what is polite. You did your duty.
She did not reply. Maybe she is busy.
Who knows.
But you did your apology.
So don't feel guilty.
You had a root canal. You could not help that. It is extenuating circumstances. Medically.
Now, you have a gift for her.
The usual way to give it to the recipient, is person to person.
That is the most, assumed thing to do.
So you call her. Say you have a gift for her. Then, you ask her WHEN can you bring it by for her?
She will tell you when.
Then, you go and give it to her in person.
After all, this is a "friend" you know from work.
So... why mail it to her?
She is in your vicinity.
If... you call her about giving the gift for her and get her voice-mail, leave a message. If she does not call back, well, the egg is on her. It is her... ball to catch and follow up on and call you.
If she does not call back, then YOU, take her gift and bring it by her at work. You said you know her from work, right? So you just give it to her at work.
That is the most common thing to do. And what co-workers will often do.