[furypt]

I've heard stories like this multiple times. Both on Android and iPhone. I don't believe it's a coincidence. The chemist is probably sharing the time of purchase along with items bought with Google, FB (probably indirectly but data gets there). These services probably have access to your location, either via your phone or via network provider. They try to match. If the likelihood is high enough (e.g. chemist had few customers at that time), show ad.

It's possible they match using location but they might have other data that increases the matching like payment provider which has a lot of your personal data as well as knows your purchased something from that store at that time.