Thank you for your message on the sample to combine with next container.
Almost of all shippers do not want us to mention the sample in L/C and in B/L, because it's no commercial value.
As for Japan side, we only need No commercial value of Invoice and Packing list for the sample (with small unit price in Invoice) for importing its sample.
If it's for commercial value, of course we need to do as the goods by container.
I'm sorry I'm not sure on exporting procedure and how to mention the requirements in shipping documents at Thailand side, so please check with your customs agent what you should do in this case.
But I think it's no problem not to mention it in L/C as we do the same for other Thailand shippers at importing the sample of No commercial value to Japan.