This time I included the size and weight when creating the listing.
When I listed the first pack (Goodall & Son), I did not include the size and weight. They are fields only in the "Advanced" section. I then went through the "Fulfilled by Amazon" which, amongst other things, asked size and weight.
This time I did fill in when creating. I went through the "Fulfilled by Amazon" and right at the start it said I may qualify for Amazon "Small & Light" scheme which is lower postage and lower fees.
I cannot figure out how I change the first deck of cards to this. But it seems it is well worth ensuring you fill in as much detail as possible when creating the listing so that things like this are available.
P.S. No, I have no idea why the dimensions have 3 values but 4 units pull downs.