Thanks for sharing your query here in the Community!
As I understand it, you’re trying to set prices rounded at 0.50 euro. Custom rounding rules are available only to stores on the following plans: Shopify, Advanced Shopify, and Shopify Plus. To provide the best advice possible, can you confirm what subscription plan your store is on?
If you’re on any of the mentioned plans, you can select from a list of custom rounding rules in your Shopify Payments settings. You must have rounding rules enabled before you can apply custom rounding rules.
If you want to include that feature in your store, feel free to learn more about updating your plan. Let me know how you get on with these steps, and if you’re able to get the rounding options there!
I just wanted to point out that this information is not correct. I am on a paid plan using Shopify Payments. The step-by-step instructions provided under customer rounding rules are not up to date, hence cannot be followed. I have just been informed by Shopify support that there is no possibility yet to add custom rounding rules. This is a feature that Shopify does not support at the moment.
If I was misinformed by customer support, please get in touch with me with instructions how to add custom rounding rules.