Similar problem here where I can't buy the potion as I can't replay the games. I also can't get the 10 special milk bottles as the interaction with the cowgirl doesn't make any text... thank you!
1. Make sure you have enough coins to buy potion "1000 coins"
2. While going back first click on back button in shop menu
3. Choose 2nd option to exit shop. (You can hold touch and drag down on options to ensure 2nd option is highlighted before lifting your finger)
Here is a video for this "I can't buy here since I have 0 coins"