As Black Horse FlexPay gives customers a revolving line of credit, they will be able to reuse their credit line for additional purchases in future on your website. FlexPay customers can also use their account at any business offering FlexPay at checkout.
As such, FlexPay on your site needs to cater to new as well as existing FlexPay customers. There is nothing further for you to do to enable returning customers to shop using their FlexPay account - this is taken care of as long as you show FlexPay and Newpay as an option at checkout.
When a customer selects FlexPay in your checkout, as well as being able to perform a new eligibility check and application, or being able to resume an application from a prior eligibility check, they can also simply login to a pre-existing FlexPay account and spend using that.
Checkout will behave in exactly the same way as for a new customer - when it is completed the FlexPay Widget checkout method resolves with a receipt object for a successful purchase, and you will also receive a separate callback notification confirming the payment.