So I think the what you are looking for is to make menu choices conditional. You can do this for both your milk and carrots and hiding choices later on. There are ways of doing this with using arrays or object attributes but for the sake of simplicity, lets just use variables. (Sorry for my lack of proper indenting, but you should get the idea)
create a variable to track if a player has made the choice or not.
default choice1 = False
make the display of the menu choice conditional to the variable (in this case only show the choice if it's the default of False
menu:
"choice 1" if choice1 == False:
Then when the player picks that choice, make sure to switch the flag of the variable to show they have picked that choice and it will not show the next time they run that menu script.
$ choice1 = True
Or in the case of milk or carrots
default milk = False
default carrots = False
menu:
"Buy milk":
$ milk = True
"Buy carrots":
$carrots = True
menu:
"Drink milk" if milk == True:
n "You drink milk."
"Steal milk" if milk == False:
n "Since you didn't buy milk, you decided to steal it."
"Eat carrots" if carrots == True:
n "You eat carrots."
FYI if you have Renpy questions you might want to try the reddit or official forums.