It won't let you remove the last card because adding a card also unlocks some more "dangerous" apps that allow running custom code. And gives higher limits.
Alternatively add a disposable one-time card. Many fintech apps provide those.
Still, as vendor, I'd avoid putting a random charge on your card and then pay the chargeback fee. Ouch.
- I'd like to know what kind of payment methods are available before signing up.
- I'd LOVE it if this supported a prepaid payment option, where the service just stops if you run out of credit. (With an email alert, of course.)