I don’t have access to Bubble right now so will try to describe what you need to do with words only.
For this you will need to create an API Workflow (paid feature). Let’s call this API workflow send_reminder. Read up in the forums/reference about API workflows if you haven’t used them before.
Your send_reminder API Workflow will need a parameter, probably for the Order in this case. Let’s call this parameter pOrder and make it of type Order. Add a step in the API Workflow to send an email to the owner of pOrder’s email address.
Back on your page, where you set the expiry date for an order (presumably on order creation), add a workflow step to schedule an API workflow (Send Reminder) and set pOrder to the order you just created. Set the scheduled date to the order’s expiry date +days(-7).