For security reason, is it possible to limit the Web Presence Builder (standalone) REST API to a few IP address (e.g. billing system)? via .htaccess or else?
I think you can do this with WPB vhost configuration. Template of this vhost you can find here: /usr/local/sb/utils/sitebuilder.conf
Also you can put right now .htaccess into /usr/local/sb/htdocs and it should work (I suppose).
Please note that zz011_sitebuilder.conf will be automatically overridden in some cases (usage of "sb_config" utility with some options, upgrades, etc). Therefore I recommend you use /usr/local/sb/utils/sitebuilder.conf template...