Totally my opinion but I'd stay away from the upstarts like M1 & Robinhood (saying this as a current M1 customer). With free trades from e-trade / schwab / td / etc. it just doesn't make sense anymore, and things seem to move way slower (for instance, I think M1 just today has their tax data integrations going live, but large shops have been set up for longer). I know some people like Robinhood and the stripped down interfaces but if you're already comfortable with interacting with the large-shop software (which I assume you are, going through e-trade and capital one), you're gonna get a lot more from that side of the market than the upstarts.
How is that possible/legal?
Stop orders execute a market trade when the price hits a limit.
So, there’s a heuristic around when “the market price hits the limit”.
If you execute too late, it might not execute at all. Too close to the limit, and it might cross back over. Also, stop orders are usually done to limit losses, so adding delay increases exposure to loss.
https://www.diffen.com/difference/Limit_Order_vs_Stop_Order
Limit orders typically have higher commissions. Maybe stop orders were being confusingly marketed as limit orders?