Magento fix checkout shipment method inactive

If you put this item in your cart and go to the checkout: [login to view URL] You will see 3 shipment methods. Problem: If you assign a zip code, click on a shipment method, payment method or select a country, the middle option (which is a 'custom shipping price') will become inactive, unable to click the option. Only if you refresh the page it becomes active again. It should be always possible to click this button. Can someone fix this asap for us? Let us know what file / setting you have changed so we can fix it on live (this is development server). Information: We use the standard shipments (configuration), Amasty shipping rules and Amasty shipping restrictions for our shipping (under Dashboard > Sales (Verkoop)) in the backend menu.