Hi,
I suppose that you must have configured something to reach that state.
For example, a negative discount that you set on you products.
And on the product page, you display the price of the product without discount, but on the checkout the price is displayed with the discount and thus you see that 1€ price for your products because you have -1 in the discount.
It could also come from the donation plugin it you had configured a custom field with the value 1 and that the plugin would then use that value as the price of the product in the checkout:
demo.hikashop.com/index.php/en/hikashop/...t-page/donation-page
Alternatively, it could also come from a customization of the views, or a third party plugin.
One thing for sure is that there never was a bug were 1 euros (or any other amount) would be added to free products (or even non free products) in HikaShop.
So it's hard to give a precise solution of were to correct this. We don't even know how you configured your products, if you have third party plugins or if you've used the donation plugin, if you have custom views, or discounts, etc. I can only recommend to check on all of them.