Weight AND region/postal code based shipping

I’m using a plugin called ‘Weight Based Shipping for Woocommerce’ and it works very well. I ship to only one country and the shipping rate generally only depends on total cart weight. However, there is one exception – There is a flat rate shipping for one specific region/postal code AND only up to 100kg. I haven’t been able to find a free plugin that handles both shipping based on specific region (not just country) AND weight.

So, I’m thinking the simplest solution would be to add a PHP code in the Checkout page which checks user data for this region and checks the total weight, and if conditions are TRUE, it shows the flat rate instead of the usual wight based calculations.

I don’t know how or where exactly to code this, or even if it should be done like this. Can someone please help?

https://www.ads-software.com/plugins/woocommerce/