Many times with some casting companies, if you order the item finished, they will not take it back.
It is best advised for you to have a jeweler order it raw, examine it to make sure the quality of the casting is acceptable, as many times they have porosity, uneven edges, and a host of other concerns, that need to be checked before you starting sizing it or setting stone, and doing the finishing work.
I''d ask the jeweler who you select to order it through what would be the circumstances, if he orders it polished and it doesn''t meet your expectations. Refund, credit etc.
You should see the item in its rough form, finished form before setting takes place, and of course when its done and completed.