You know, there's something to be said for just telling readers the facts straight off (though often that can be uncomplimentary somethings). There is no point trying to lift this sentence to make your own work better because it works HERE (and when people borrow things those borrowings often stick out as different from the rest of what they have).
HERE is a well crafted story which in the end tells us Marie's probable future using the advertisements.
THERE WILL BE SPOILERS - ONE BEING THAT I LIKE THIS STORY - GO *READ* IT AT
Advertising at the End of the World by Keffy R M Kehrli
I can imagine ads --I'm not sure that they'd cost less than people being sent door-to-door but I can imagine them being more acceptable, the way many people find ATM machines preferable to cashiers. An ad can come in your house and probably not be a thief pretending to be a salesman, and may use pressure tactics but probably won't assault you. Robert's theory on why they exist doesn't have to be correct, any more than his wondering how sentient they are -- although that did open the possibilities delightfully while I read, so that the ending was being written from the beginning but not without options (until you reached the end). I wondered, like Robert, if the ads could learn, would develop and replace their dead makers. I wondered if Marie could make use of them. I wondered if they might wander the land, desperately seeking someone to sell to. Or if Marie could pin notes to them and they'd find other survivors for her.
That's not a story problem, that's a delight, when you parallel process with the writer and are keen to follow and find out what does happen.
The narration is strong, it leads you on, but like a river -- with elegance and a perfectly measured pace. It unwinds.
And gradually the backstory is revealed, the ads understood, and at the end as they die you realise that Marie, without her pills, without others, with a frail human body of finite energy, will like them fail and die, or power down and die. There is the faint hope held out, that maybe there are other survivors, and maybe she can find them and share the burden of survival. But equally the knowledge that she may gradually grow to slow to garden, too tired to forage, eat her stores, and fade.
Quietly I realised that the ads are Marie, and the ads are humanity -- threatening in crowds, and destructive, and useful only to a point -- so easily running out of life and then just bodies to clear away.
This is a lovely, simple, human, story that I suspect will stay with me long after I've forgotten it's title and where I read it.