If you have Shopify Plus, you can access the code to checkout pages, and when there if the order is above 150 euros you could add a message and prevent the custom from advancing.
If not on Plus, you can add some logic to your product pages and cart pages.
Here’s what I would do (you’ll probably need a developer):
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Add some JavaScript so that when someone clicks on add to cart button, it checks the contents of the cart + the price of the product, and if it’s too high prevent the add to cart action and trigger a warning message instead.
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Add similar logic on the cart page, preventing them from clicking the checkout button until they lower the contents on their cart, just in case they were able to add more to the cart (like if from an app or something).
That would be my plan B.
My plan A would be to do whatever you need to do to remove that 150 euro limit. You’ll be leaving money on the table by having a limitation. I’ve owned a store and even with low priced products have orders in the hundreds, and some in the thousands of dollars.