You can buy directly at Chartworld - or more precisely license
periods are either 3, 6 or 12 months during which you will receive updates for the charts. After this time the charts are still usuable, they will not disappear.
You will need the UserPermit you got already so that Chartworld can issue charts for you.
As always in the case of S-63 the charts are only valid for one type of systems, in our case OpenCPN and it's S-63 plug-in.
For your individual machine/PC you will need an InstallPermit issued at the o-charts site.
We recommend to study the documentation
from the o-charts download page, S-63 explained and the How-To video.
We know from people that licensed directly from Admiralty, Primar and AHS (Australia) but in this cases we have no feedback.