Subscribers manage their subscriptions on the Subscriptions page of your Shopify customer account. You decide which buttons they see there. Anything you turn off simply disappears, and the customer has to contact you instead.
Go to Apps → Progus Subscriptions → Settings → Customer Portal. The switches are grouped into three sections. Changes apply to every subscription in the store.

Under Subscription actions you control what a customer can do to the subscription as a whole: Pause subscription, Resume subscription, Cancel subscription, Edit delivery frequency, Edit delivery method, Edit shipping option, Edit payment information and Add new subscription, which shows a button back to your store.
Under Subscription product actions you allow or forbid changes to what is inside the subscription: Product swap options, Add products, Remove products and Change quantity, which lets a customer take more or fewer of a product each time at the price they already agreed to. The add and remove buttons also appear when Add or remove one-time purchase products to upcoming orders is on, even with these switches off.
Under Upcoming orders actions you control the next deliveries: Order now option, Skip upcoming orders, Reschedule upcoming orders, Add or remove one-time purchase products to upcoming orders, Edit shipping address for upcoming orders, Show store credit balance for upcoming orders and Edit payment information for upcoming orders.
Below all of it sits a Limits section, which takes capabilities back rather than granting them, and a minimum commitment before a customer may cancel. "Limit what customers can change" covers both.
Click Save. Seven switches are on out of the box:
Pause subscription
Resume subscription
Cancel subscription
Edit delivery method
Edit payment information
Add new subscription
Skip upcoming orders
Everything else is off, so switch on what you want before you tell customers about it.
These switches govern both billing engines. Nearly all of them reach a Mollie-billed subscription too: pause, resume, cancel, skip, reschedule, frequency, swaps, adding and removing products, quantity, one-time extras and the shipping address, each only if you switched it on here. Three behave differently.
Two do nothing: Edit delivery method and Edit shipping option read delivery options off a Shopify subscription contract, which a Mollie subscription does not have. The screen says so next to each.
Edit payment info works differently rather than not at all: there is no stored card to pick, so the customer authorises a fresh SEPA mandate. That one authorisation covers the next order too.
Allow store credit is the third: a Mollie customer sees their balance but cannot spend it, because those orders are charged by direct debit rather than paid at a checkout. The card says so.
Their card is simpler than a full detail page and follows the status: paused shows only Resume and Cancel, and one still waiting for its mandate only Cancel.
Activate the customer-account blocks so the page is reachable at all.
Set up a cancellation flow if you turned cancelling on.
Check the upcoming orders card, which is where skipping and rescheduling actually happen.
Stuck on any of this? Open the chat in the bottom right corner of the app and we will help you set it up.