Those guidelines seem to be more concerned with packaging wastage (i.e. too much packaging for a little product, recyclability etc.) or heavy metal concentrations, can't see anything in them about sturdiness of the packaging (unless the "consumer acceptance" is taken into account).
In my humble opinion, from a very basic level this would be covered by section 2(2)(b) of the HASAW etc act 1974 on the part of your customer
arrangements for ensuring, so far as is reasonably practicable, safety and absence of risks to health in connection with the use, handling, storage and transport of articles and substances;
It is up to them to ensure the articles (once in their possession) are stored, handled etc safely, and if they have any concerns they should relay them to you for you to action under section 3(1) of the HASAW etc act 1974
It shall be the duty of every employer to conduct his undertaking in such a way as to ensure, so far as is reasonably practicable, that persons not in his employment who may be affected thereby are not thereby exposed to risks to their health or safety.
So basically if you are ensuring your goods are arriving in a safe and healthy manner (SFAIRP) (so no sharp edges, toxic materials etc) then you are fulfilling your duty. The very fact that this packaging has been suitable for you to move around when unloading suggests that it is fine.
As an example, when we deliver goods, it is up to the customer to unload these goods, not us. We ensure they arrive in a safe condition, and if the customer disagrees they are to return them to us, and we make them safe again for delivery.
If this were not the case then it would seriously confuse all manual handling claims. If somebody hurt themself at work, who would they claim against? The company for not ensuring their safety, or the supplier for supplying goods that were too heavy?
Maybe my inexperience has me simplifying this too much?