Only pay highest flat rate shipping?

8 0 3

I am struggling with the shipping options to make it work for my store.  I have 2 products...1 is small and fits in a flat rate mailer.  the other is big and requires a shipping tube.  I have it setup fine if someone buys one or the other.  If they buy one of each though I need to use the larger tube and ONLY charge freight for that tube. Is there a way to acheive this without an app?