Use CRON jobs to send email at anytime in the future. Specify a specific and/or relative date to give you the maximum amount of flexibility. You can even configure a parcel to send emails at re-occurring intervals.
This is directly from their Devot-ee page. You need to set up a cron job to curl your site once a week; It seems PostMaster allows for triggering emails via a URL. I'm busy and wish I had time to dig through their documentation, but I'm pretty sure you can just make a template, and you curl that path:
and just curl that with a cron job, and use something like
to actually send the mail. You can probably use an htaccess block on that template page to only allow request from 127.0.0.1; then you're protected and only your server can curl that notification address!
Note Read through the docs. I probably just gave you a more complicated solution than is necessary, the people at Objective are quite nice (in my experience) so you can just shoot them an email and they can probably walk you through it.