Some Customers want us to “Keep Their Card on Fil”

[Gwhatever]

Are there any “keep my card on file” apps out there? Shopify still doesn’t seem to offer a payment storage function other than recurring Subscription services.  
We have a high client base that wants to be able to submit a Purchase Order and be hands-off beyond that.
We shouldn’t have to also process payments through Quickbooks if we are already operating inside Shopify. 

any insight would be very appreciated!