Hi Van,
In WooCommerce, when you define a price, you define it per 1 unit, so when you mentioned it’s 3.59€ (if that’s in the backend of that product), then it means this is the price for 1 KG and not 250G.
First thing, I suggest you update the plugin, the version you have is very old, you can download the latest version from your account page here on WPFactory then upload it to your site.
Second, send us please screenshot of price settings you have for that product, and link for it as well please to [email protected] and we will be glad to assist further.
Omar