What about a singular note? If I’m sending her notes does it become la? If I was sending them notes would it be las or les? If notas is feminine why don’t I use la?

I don't know spanish, but based on my knowledge in fr*nch, I think "le" returns to the person being sent, since it's a known person "the person" and the notes are "notes" "a note", for them, it depends on the gender of the group you're sending, a group of men it's les, group of women it's las, group of men and women prolly les too. You don't say "la notas" because it's multiple "a note".

Is there a simple way to remember whatever the rule is? Thanks

The issue is that you're probably using English direct translation to try and understand it, if you're an English speaker you should know that English isn't really a gendered language. I guess a way you could do it is making the sentence like longer, from "I am sending him notes" to "I am sending the male person notes"