sub.club seems to be a Mastodon fork with some additions. In particular, there's a subscription object attached to the actor:

"subscription": {
    "currency": "usd",
    "interval": "month",
    "intervalCount": 1,
    "paymentType": "recurring",
    "unitAmount": 300
  },

In Mitra this information is represented as FEP-0837 'Proposal' object, which is attached to the actor using a FEP-0ea0 link.

I asked if they could implement that.