To instantly charge a Keap Subscription using Instacharge, simply add on an optional POST parameter subscription with a value like true.
That will tell Instacharge to look for a subscription instead of an order and charge that accordingly.
This is especially helpful if you have an Automation in Campaign Builder or an Upsell that creates a subscription for your customer as Keap only generates invoices for new subscriptions and charges them every 6 hours.
Instacharge will immediately find the subscription, generate the invoice, and bill it using the Merchant Account you specify.
The success or failure of that charge can then kick off the right automations in Keap, for example to welcome a new member to your membership site, or send out a billing failed email to get updated card information.