Yes, of course. You can pause or cancel a subscription any time you want in the customer portal, as seen in the screenshot below. The only time when the subscription can't be cancelled is if you started a subscription which requires a certain number of payments before you can cancel it.
The customer portal can be accessed through the magic link you received in the email that was sent to you when you placed an initial subscription order in the shop.

You can skip or reschedule the charge in the customer portal. Each scheduled delivery will have a Skip payment button next to it, which you can use to skip the charge. You can reschedule the charge by clicking on the Reschedule button next to the delivery date.

Of course. You can add products to your subscription in the customer portal. Just click on the Edit button in the products card and edit, remove or add products to your subscription.

If the subscription can't be renewed because a payment can't be processed you will receive an email with a link where you can safely update your payment method.