Walking into any shop, cafe, or pharmacy without saying 'bonjour' first is genuinely rude. It's a hard-coded social rule.
Even at the boulangerie counter, start with 'Bonjour madame' or 'Bonjour monsieur'. Then ask for what you want. Skipping it is interpreted as dismissive.
At the end, 'merci, bonne journée' (thanks, good day) or 'au revoir' closes the interaction. Adding these tiny rituals will visibly improve how French people treat you.