In my experience, shops you frequent will be sympathetic to that problem and will just let you come back to pay later. And I imagine that any shops that sell stuff that needs to be kept cool were especially amenable to that ...

I spent my last 5 euros on a sandwich and a croissant to eat on the walk home. All in all a pretty decent experience.

Your comment immediately brings to mind this classic of croissant eating in ‘interesting’ circumstances: https://youtube.com/watch?v=wp84sRpM1Js

Add the store on Foursquare, "good blackout sandwich, 5 stars"