Thank you. Yes, ShapeShift does have this limitation.
I did not provide enough details in my previous post.
I do plan to add an instant market order in the near future. Users of the exchange can then have a choice of limit or market orders. An instant order will either execute (possibly with some slippage) or fail if there is not enough liquidity. It does not matter if the coins are fiat or crypto, the implementation would work for all coins. The instant market order will not be part of the next release of the Exchange, but will come soon after.
What is often requested, however, is to “package” this into a nice UI where people can send coins to an address and receive them on another address at the other chain. This is what services like ShapeShift do, and this would be an extension of its own: It would have to expose an API, make some kind of estimate/guarantee to the user about maximum slippage, and handle any failures gracefully. Thus I don’t have time to develop such an extension at this time. Perhaps in the far future.
with regards