I'm not sure where the rules come from (law, business association regulation etc), but here are a couple of links for the UK. https://www.lawdonut.co.uk/busine

I'm not sure where the rules come from (law, business association regulation etc), but here are a couple of links for the UK. https://www.lawdonut.co.uk/business/marketing-and-selling/consumer-protection/unfair-trading-trade-descriptions-and-pricing-faqs https://www.lawdonut.co.uk/business/marketing-and-selling/co... https://www.lewissilkin.com/-/media/files/main/insights/cmi/important-new-guidelines-for-advertising-prices-to-uk-consumers-2.pdf https://www.lewissilkin.com/-/media/files/main/insights/cmi/... You'll find similar rules in many European countries, though most won't be available in English. This document is, for Denmark: https://www.consumerombudsman.dk/media/14560/forbrugerombudsmandens-retningslinjer-for-prismarkedsfoering-awfeng.pdf https://www.consumerombudsman.dk/media/14560/forbrugerombuds...