I suppose it depends on what you want to have in your store. If the store is going to just be for a few items (like PCBs) then that seems pretty simple to know what will fit. I might go one step further and just take the largest item and use that to calculate.....Then no matter what people order the shipping container is based on fitting that many of your largest item.
This is like the Quality, Time, Cost triangle.....Pick two.
If you want it to be inexpensive and highly flexible (figures out shipping real well) it is going to take a long time (because you are probably going to need someone to volunteer their time to develop one for you).
If you want it to be inexpensive and available soon you are going to have to sacrifice flexibility (with something like I described....everything is the same size, even though that isn't true).
If you want it flexible and available soon then it's time to spend the money for a commercial product.
Just my take. I think most of us would get along with being on the hook for a little extra shipping if we were in a bind and needed to order something now.
I like the dealer idea too....allows you to spread the work some, but with adding more shipping not sure that is any better than just assuming a certain size from your store.