What's behind the EXRTRA risk? Couldn't he just simply pass over the liability to Stripe who's the actually payment processor?
We had an accounting software that was used in some pretty big Martial Arts schools, we handled their payments through Authorize.Net (stripe didn't exist yet), but in the contract, we told them a reversal will cost (making the numbers up because it has been so long) $25 + the transaction and a bounced eCheck was $30 + transaction. These fees were directly pass through, we didn't try to profit on them.
That said, the number of fraudulent card/check or bounces was 0.
We were the payment processor for a business, not working with skeezy customers directly (like Duet is).
If there is fraud, you bill that back to YOUR customer. Just like Stripe bills you for a bounced check or a reversal, you bill your SaaS customer.
BadGuy uses your service to sign up for BadGuyDojo, and attaches a stolen bank account. 10 "customers" sign up with stolen credit cards, process payment, and the money is gone along with them. A month or two later the credit card companies come looking to you for the money.
I'm a small business owner and you are my client. You and I have a relationship. You aren't a random BadGuy. The likelihood of you purposefully bouncing a check or committing fraud is next to nil. In this scenario, similar to what Duet is doing (facilitating communications and payments between known vendor-client relationships) the SaaS that processes the payments is pretty safe from fraud.
Now, I would never roll this out on any of our SEO services. When we had credit card payments on the earliest version of Linklicious, we fought over $1200/mo in charge backs from fraud or "fraud." After nearly 3 months of our mailbox filled with chargebacks and complaints, we switched to PayPal and only once did we have an obscene amount of fraud (hundreds of signups from stolen French PayPal accounts) at which point we blocked the country of origin for a week and the problem disappeared.
You have to be intelligent no mater what payment method you chose. There are negatives on both fronts.