Intuition says: "I send few campaigns, this doesn't affect me". But the change doesn't touch campaigns — it touches replies. And in a portfolio that works, replies are most of the messages.
The usual pattern: you send a due-date notice to a thousand people. Two hundred reply. Each has a conversation of three, four or six messages: asks the balance, says they'll pay on Tuesday, sends the receipt, asks to change the address. Today all of that is free. From October, each of those messages is paid.
Put another way: next month you'll be paying for the part of the service that costs you nothing today, and which is precisely the part that makes it work.
+ + One more change, in force since August 2026
If you use Meta's own AI agent, its replies also carry a usage charge, billed separately. It's another product and another invoice. It doesn't apply to assistants running outside Meta.