Apps to use in Prague: Rohlik.cz. Rohlik is a grocery delivery service, with a great selection of goods (including grass-fed meat & organic produce) and perfect reliability. You can specify 15-minute delivery windows, and these are usually kept (unless a traffic jam happens etc).
In fact, some people in Prague (like me) never go to physical groceries anymore, as Rohlik is so convenient & reliable.
Very useful if you're staying in an Air BnB and want to cook for yourself from high quality ingredients.
Courriers accept cash (or cards).