Issues with multiple-product orders at checkout

[CC02]

Hi,

So here's the scenario: Customer places item A and item B into their cart (two different products), but they are met with an error message and can't finish checking out. The error messages pertain to the shipping details for the product(s) in question. However, I have set the shipping rates for all of the individual products on my store. This is only occuring when customers try to pair two unique products together. Any help would be greatly appreciated. 

Thanks

[Zqdo]

What is the error message that shows? And when exactly does it show? Is it after they click checkout?