Allowing Customers to use their Shipping Account

[rtlslab]

All,

I'm hoping someone has sorted this out already. 

I need to allow customers to use their own shipping accounts and $0 the shipping charges on Shopify?

This seems like an obvious feature but there is no add-on / plug-in for it and the only advice from shopify support is to write my own through an API. They must get a good order-flow kickback.  

It is my bad, but Shopify has turned out to be a waste of effort for us without this capability. 

TIA