Our shipping is currently setup using 3 categories each with custom flat rates. The problem we are running into is international vs. domestic. The ideal solution would be to add an additional fee (lets say $15) to ANY international order in any category. This would also apply if a customer ordered multiple categories of products, $15 would still be added.
My first thought was to use tiered shipping as mentioned in this thread http://forum.foxycart.com/discussion/6545/international-shipping
but their situation is slightly different because they offer free domestic shipping, we charge for domestic and it's different in each category.
I hope this all makes sense, any ideas how I can achieve this?