Hey there!
For your audience, it sounds like you can just set your inclusion filters to be based on Membership Name, and Membership Status (in this case "Active" will suffice). Cadence wise, our automated campaigns are only able to have 60 days between drips so what you're trying to do in one campaign will not be available. I would suggest doing a 1x campaign for the semi-annual sales and name them specifically (IE: July 2023 Semi-Annual), etc.
This might make great sense in general seeing as you'll likely want to add specific expiration dates on the postcards themselves to add urgency for usage.
In the meantime, I highly encourage you to post in our ideas portal for a feature improvement to increase the cadence time between content drips from 60 days!
