Change a custom magento plugin: change shipment price "on the fly" -- 2

I have a custom Magento 1.9.* plugin where a user can select some shipment options on the Magento Checkout page. But some options have a custom fee (extra shipment price). As example: when a shipment price is 5$ and a user choose the option "weekend delivery", the shipment price should be increated to $7 (as example). But how can I change the shipment price "on the fly" on the checkout page, ONLY for a certain shipment method?