As an addendum to anyone who finds this.

After an extended back and forth and them temporarily marking my account as active so they could look around and try things, linking the accounts by logging into Amazon and One Medical in Chrome worked.

I generally use Firefox these days, but was pretty sure we had tried Firefox, Chrome, and Safari at one point previously in troubleshooting, so I don't know if some machinery on their end changed or I'm an unreliable narrator, but I thought I'd mention this in case anyone else finds it.