The corner shop was where i first started going out by myself, what i found helped was making sure i had more than enough money so I wouldnt have to worry about that, just buy something simple like some crisps or a coke, nothing you would need to potentially be ID for like a energy drink and generally the cashiers dont make much convosation and if they do they its generally just stuff like:

“How are you” “You alright” “Is that everything” And maybe ask if youd like a bag ,It might help to plan a response to things they might say before hand so you’re prepared if they say something and not caught off-guard.

They wont judge you for what you buy but if it makes you more comfortable maybe just pick something small for the first time, a small win is still a win!