I never heard that there is a DeFi-based cryptocurrency payment gateway why not tell us the any of defi project that you know have this feature?
For me, I don't we need this as we already have a crypto payment gateway that is open-source, free, and decentralized so why would I need to use their defi project if sample using BTCpayserver as a payment gateway it is decentralized and all of the invoices are generated from your own wallet that you fully control and it does not have 3rd party unlike you said defi based payment gateway I think it still needs to bridge meaning it will still use a middleman or a 3rd party unlike when you use BTCpayserver.
Sorry, I thought I mentioned the project, my bad. It's name is Paymento (paymento.io), I found them on LinkedIn. I don't know if they're open-source but yeah it says that it does the way you described here. I think Paymento has a Buy Now Pay Later feature? either that or they're working on it for the future. here's the post on LinkedIn:
https://www.linkedin.com/posts/paymento_paymento-cryptobnpl-innovationinfinance-activity-7202320584066691072-3kZV?utm_source=li_share&utm_content=feedcontent&utm_medium=g_dt_web&utm_campaign=copyI've done some research in the meantime, and apparently crypto payment gateways are custodial. I'm looking for a non-custodial options and wanted to know if even it is possible or not (I'm not very into the technicality of crypto, I just HODLed some in the recent years and now I want to integrate some gateways with my website, so I'm asking around)