If you encounter with this issue

The link you have used to enter the PayPal system contains an incorrectly formatted item amount.

Please bear in mind that this is not a GoodStore issue but Woocommerce Shipping settings. In the URL you can see amount_2=XX,XXXXXXXXXXXXXXXXXXXXXXXXX (paypal accepts only 2 decimals, e.g. X.XX).

How can I fix it?

It is caused by Additional Rates in (Woocommerce->Settings->Shipping->Flat Rate->Additional Rates). When you remove “|Yes”, it works. But we strongly recommend you follow this tutorial