How can I get shipping rates from Checkout? (JS Buy SDK)

Polymath
Tourist
5 0 2

I searched a but for similar posts - so apologies if the answer exists here someplace. 

I've got a custom website hosted outside of the online store. It uses the JS Buy SDK. Right now, my customers checkout flow looks like this: 
- Customers browse my site and add items to cart (using the checkout functionality of the buy SDK)
- They review their cart on my site 
- When they click checkout, they are sent to Shopify checkout with the webUrl

I've got quite a few products that cannot be shipped to certain areas though, so what happens is sometimes they end up in the checkout flow with no way to get back to their cart. I want to find a way to present shipping options to them through the Buy SDK, and only send them to the checkout once I know for sure all they need to do is pay. 

It seems the best way to do this is to have them enter their shipping address on my site, and update it through the SDK - but the checkout doesn't seem to return any shipping items when I do this. 

What is the best way to get Shipping lines to present to my customers on my site?