Thanks Mohamed!
Okay, let's see what I can tell you.
Shipping settings first.
There are two shipping methods, one for shipping within Germany and one for worldwide shipping.
Each product specifies custom prices for both shipping methods.
Settings for shipping within Germany: See attachment shipping-settings-germany.png
Settings for worldwide shipping: attachment shipping-settings-worldwide.png
Next, here're the settings of the first product for which I created variants.
product-settings.png
Let's continue with the settings of variant 1 (set to default variant).
Noticed one unusual thing here: In Shipping prices box, nothing is added.
And if I try to click the plus sign, the view will jump to top instead of creating a shipping setup row.
product-variant-1.png
In opposite, Variant 2 specifies shipping prices. See product-variant-2.png
If I set Variant 2 to be the default variant, it's shipping setup will disappear.
So far, things feel a bit messy but alright.
However, I did add these items to cart next.
If I add Variant 1 to card (default variant), shipping cost will be correct: 5 € / 11 €, just as expected. cart-result-variant-1.png
But if I add Variant 2 to card, shipping cost will double: 10 € / 22 €! See cart-result-variant-2.png
I tried to delete Variant 2's shipping prices hoping it would work the same as the setup of default variant / variant 1.
But these where added back in automatically.
Next I set them to 0. But this resulted in free shipping.
Now, I'm pretty much clueless what's wrong here.
Hopefully you'll have a way to make this work again
Thanks!
Jannik