I think you are also failing to mention that your service has to be at a scale where these partners care enough about you to lift a finger. Wix is big... huge... and shipping companies will be willing to put in some investment to ensure that stores built in Wix can use them as a shipping option. I guarantee that if I built my own ecommerce platform today with 0 users, and built an SPI for, say, credit card processing, the merchant gateways would laugh in my face if I did this... but if Wix did, they'd at least discuss it.

Not to say "SPIs make no sense", but saying "let's be realistic". Few of us are working on projects with enough juice for anyone to invest any resources into this kind of thing. :(

J.Ja