$LJHOME/ext/local/etc/config-local.pl (copy it first from
$LJHOME/etc/config-local.pl if it doesn't already exist) to uncomment the variable
%SHOP. Note that it's multiple lines to uncomment, but don't accidentally uncomment the
key => line which is a comment.
You will also need to install the CPAN module
Business::CreditCard which is not checked for by checkconfig.pl and may not be installed.