Oh what a tricky one. I think cover all bases:

1. Give her the 'what you're going through is normal' chat and show her how to use the pads. Tell her they should be comfy, if they're not, she's putting them on wrong.
2. Gently explain that everyone has accidents now and again and you don't mind washing her undies when that happens BUT not when she's made no attempt to wear a pad/tampon. Also explain that after a while blood smells and it will show through her clothes. When that happens, she is likely to be teased.
3. Tell her that if no pads are worn, those undies don't get washed and she will have to wash them herself. If she doesn't do this, eventually she will only have dirty undies.
4. Tell her that she will NEVER have to buy her own pads/tampons if she's embarrassed, that you are happy to buy them for her and she can keep them in her own room.
5. Ask her if she would like to come shopping and choose them (with you paying at the counter so no need for her to be embarrassed).

I do sympathise - my 14yo DSD did wear pads. Which she then left (used) on the bathroom floor. I'm sure Freud would have had a field day with that one!