I'm in the process of figuring out How To Patreon, and I think I have a rough idea of how things are going to work. To wit:
1) Draw comics at least one update ahead.
2) Post the strip image file to the Silver Shilling patron feed as an unpaid Creation as soon as it's done.
3) Post a link to the strip as a public paid Creation when it actually goes live on the site.
This means that Silver Shilling patrons (and above) will get the strip early as promised, but won't get charged for it twice.