The lesson
When an account goes quiet, have an executive send a plain note naming what you saw, asking one thing, and paying for it.
Nick, who signs as VP, Revenue & Customer Experience, writes "We noticed that you're using your Mercury account for Magpie Mentors LLC less or that you've moved funds elsewhere." He asks for a short survey and offers a $20 gift card, with one dark Start survey button.
Don't copy the line about usage dropping unless you can state it accurately for each recipient. Keep the single ask, the incentive stated plainly, and a signature from someone senior.