Even when I was selling through Kagi, I had a backup payment service running simultaneously (Plimus, then later FastSpring). I could switch between them in just a couple of minutes by changing some if statements in my PHP code. I even had hidden 'coupons' I could give to customers to redirect to another provider, if Kagi wasn't accepting their card for some weird reason.

Don't hard-link to a third party service. Abstract everything through your own site that you have 100% control over, so you can swap out the underlying services whenever you want.