Why is there a full list of delivery/pickup days for my customer?

You are only seeing that full list because you are logged in as the admin.
From the customer end, they will only see the delivery days you have made available for this/next week. 

To double-check, you could go to your menu in a private/incognito browser (so you will be logged out) and you will see it exactly as the customer.

Don't hesitate to contact GoPrep's Customer Support Team for further assistance.

You can do so by submitting a request or sending an email at [e-mail].