Shipping based on items in cart question

I have 2 sets of products one that I ship on my own and another that is shipped by a service I use. They have a shipping plugin that only works with their items in the cart so when I add one of my products it does not show their shipping options only mine.
The problem is when their products are in the cart it shows my shipping and their shipping options How can I hide my shipping options (Flat rate)?
Or how can I make the cost of shipping increase when more than one item is in the cart?