Andy, I understand about being conflict-avoidant, but the simple fact (As I can see from what we read here) is that you were not supplied with what you paid for. You paid for a specific kind of wood, expecting a certain level of quality and consistency, and this was not delivered. For the kind of money you are investing in this project, you should in no way have to make compromises to your build because of a supplier's inability to perform quality control. Even though it will work, you are making a compromise in your planning and execution, complicating an already difficult project. As an amateur builder, you also may risk compromising the performance of your boat if, by chance, the balance isn't right. Again, because you are working around a vendors quality issues. If they are not willing to exchange the wood, they should at least be willing to credit you the amount of wood that was incorrectly and neglegently included in your order.
In these cases, never ask "if it would be okay". Especially if you are communicating via email, simply and respectfully demand that this is unacceptable. Show that you are willing to work with them, but at the same time, demand satisfaction.
How about:
Thanks for your response. In retrospect, I can't see how I can use the heavier sheet material without either over-complicating the execution of my build or possibly risking the performance of my boat by making a mistake in balancing. I cannot accept a mixed shipment. Please contact me with a plan to replace the erroneous material or credit me the difference.
See how that goes.....
It's amazing the damage that a bit of free BAD advertising about a company can do.....
Sorry, got carried away....

_________________
Ken Hundley
http://www.nocturnalguitars.comSo, my big brother was playing guitar and I figured I'd try it too.
- Stevie Ray Vaughan