Amsterdamsche Kiosk Onderneming is a stationer's. It, currently, calls itself AKO.https://www.ako.nl/
It, originally, sold dailies to travellers at railway stations. So, the "K" for AKO and its connection to railways should not be a surprise. It, likely, was stock used at the stationer's at stations.
My guess would be Rotterdam Maas Station. Up to 1953, "Maas" was the station for trains from Rotterdam to Utrecht. Now, it is the Central Station.
Could it be signed "van uw Oom Tante en Stientje?"
The name J. Heuken, I think only appears (in a different ink?) in the lower left corner.
D**str 18a (D** street 18a)
Rot in the date might be Rotterdam. Steenwijk is East of Utrecht, Rotterdam west.
"De bloemetjes buiten zetten" can mean "painting the town red."
However, if the picture postcards were sent by people that settled in Steenwijk that sent picture postcards of their new town, it might also literally mean to put out the flowers.
The date stamp appears to be for February. Spring is coming. People will soon (March) put out flowers they kept in the house to protect them from wintery conditions.
Both interpretations are possible.