To create a package or a subscription, go to Memberships > Subscriptions/Packs and then click + Create. Choose to create a package (or subscription), then set its name and price.
Choose how many credits the customer will get. (Consider what you've priced your services at when adding this figure).
Then, under "What this can be used for," click on "Appointment services that can be booked" and select specific service or all of them.
To make it publicly discoverable, from the memberships index page, toggle on "Featured at checkout."
Now any customer that's self-booking an appointment service included in this pack will see an option to buy this pack at checkout.