You can just create a new 'price based' shipping rate for $250 and up with a shipping rate of $0.00. That will allow you to give free shipping to orders in your shop over that price.
Here's the relevant doc for your perusal:
Let me know what you think!
I think this step by step guide on offer free shipping over a set dollar amount may help you solve your issue.
Hope it helps!
@skonrad pretty much what the others said. some docs over here:
If you're on S+ you get some more flexibility with Shopify scripts too: