Take only highest shipping rate if combined rates

Shopify Partner
122 9 41

This is an urgent issue, I can't believe there is no solution for this. I guess I'm wrong, as this is too basic.

I want to sell postcards with a cheaper shipping rate, and all other products with a higher standard shipping rate. 

If someone is ordering both, postcards and standard bigger products, for sure, I just want to take the higher shipping rate and not have the shipping rates added up.

Totally normal stuff right?

How can I do this?

I do NOT want to buy any app for this!!