Had you been trying to purchase anything?
It looks like you were using a browser at the time. I've not heard of an ad that attempts to buy something, but if this only happens when using the browser I'd clear your browser's cache for certain and see whether that is sufficient. I'd also ask whether you have installed or updated any apps shortly before this started? Especially any that have the permissions to make in-app purchases.
No, I've not seen this before, so just thinking about how you might work out what is responsible.