Does anybody here know where I could update/enter the url path for Affirm to their sandbox url?

You mean the affirm JS url path?

that's what's confusing to me. I thought Solupay will run practically out of the box scratches head. no one is cooperating to make this Affirm work!

Does the sandbox URL not just get used if you use sandbox API keys? https://docs.affirm.com/Integrate_Affirm/Platform_Integration/NetSuite_SuiteCommerce_Advanced_Integration

Affirm has two components. 1) the payment integration. That works with solupay and sca’s external payments just fine. You don’t need any extra code for that. You just set up the keys in solupay’s payment processing profile. In a sandbox, with sandbox affirm credentials (and solupay in test mode), i'm able to process orders just fine. 2) but then, you might want to add affirm marketing (“affirm promotional messaging”) to your shopping and cart pages. Like the affirm logo on PDPs with “from 15 doll/mo with Affirm” that when you click on it shows you an affirm informational modal with calculator. This doesn’t come with solupay’s integrator as far as I’m aware. But is by far the simplest piece of the integration, compared to payment processing (which solupay solves for you)