I use Paypal credit and it does 4 months interest free if you spend more than £150 (in America it’s $100) its almost like curve is the card version of paypal.
I could have a credit line say £1000 and i then get a monthly statement which i can pay minimum payment or higher if not paid in 4 months then intetest would apply.
Please bear in mind paypal is accepted on a lot of retailers and there is no such thing as pay by curve option on websites. Gather you would have to use your curve card directly.
However the advantage would be you could use it anywhere and not just where paypal is accepted.
The credit line could be where your reward card is and you just select that card. You then get emailed a statement to pay it off monthy with an intetest rate. Say 14.9%. The problem is would people not use underlying credit cards anymore. If you haven’t got paid yet then you simply pay it via your cards in curve.
However you could undercut paypal by having a spend £100 and get 4 months to pay it off option.
In a way i feel if paypal launched a 0%fx card then it is pretty much the same as curve