Take education out of it. This is a service that OP is paying for that other people, who don't pay for it, are not entitled to. My mom lives in a neighborhood with community pools and parks. She pays HOA dues to maintain the pools/parks. My dad lives in the neighborhood just north of hers, but his neighborhood doesn't pay into the pool neighborhood HOAs. It would be wrong for my dad to go to the pool in Mom's neighborhood, even though he lives so close, or he can get to it, or Bro gave him a key. Mom and her neighbors pay for the pool.
I have a zoo membership that I buy every year. I can take guests, but I can't give SIL my card and let her go to the zoo with my pass. She didn't pay for it, and she's not entitled to use it. I realize these aren't exactly the same since Mom could
invite Dad to the pool as her guest, or I could invite SIL to the zoo as my guest, but it's the best I could think of
OP, I think that if you are going to report the family, just do it. Don't contact the woman, just go to whoever is in charge and outline your concerns. I would keep it to the bare minimum and just the facts (she uses a drop box, possibly conversation you had with the person at her listed address.)