When dealing with a commercial company, you may not get a clear answer because they do not understand the requirements. Companies generally use the cheapest plasticizers and raw materials available. This is especially true for products like plastic bags as they are not intended to last long. So, it’s buyer beware. Also, some information is proprietary and will not be shared. Another drawback is that the commercial market is consumer driven and changes to composition can change without notice in order to please market demands. The answer you get today may not be true tomorrow.
If it is important to have a good quality box, I recommend simply asking if a company’s products meet the appropriate ANSI/ISO/NSO Standard. If it does, they will know. If they don’t know or have never heard of the product, then you may well be buying short term relief at the expense of long term grief.
If price and not quality is the deciding factor, I recommend doing the best you can with what you’ve got. If a box is required to protect an artifact, any box is better than no box. If there is already a box, a good quality box is better than a poor quality box. There is information readily available on making enclosures, and I have seen successful rehousing projects undertaken by summer students. Group purchases can also bring the price down.
Just some thoughts. Thanks for the opportunity to share!