Just tried to buy the next snp pack to take my DNA further and it says I don't qualify for this pack. Am I doing something wrong? Can someone advise me?

With closer look it seems that they have found 20 common SNPs from two kits (id:HG00105 GBR and me) and chosen one from there for their haplogroup name?

Simo,

Here's what was shared with us by FTDNA management as to the content of the Z253 SNP panel. I suspect that some additions may have been made, but I do not have that chart/list.