I haven't been able to StartPurchase() with "steam" store at all. It's pretty straight-forward, all I need to provide is "en" for English, "steam" for the store name and "someItemId" - that's it.
I get a vague err response that doesn't seem to assist me in any form, and with err code 0 (that doesn't exist as a Steam err code). This has been the most frustrating experience trying to get this working since I can't even self-diagnose with vague errors
Since it's actually incredibly easy to init the purchase from the client-side, this seems to be a BC internal issue. Their docs also have no mention of Steam at all - and the only article I could find pretty much shows what I already implemented.
I had it working on GameSparks no problem (after implementing the entire flow manually, myself), so I'm very familiar with the process.