Associate shipping methods with stores

[mdavo6]

Adds an association between shipping methods and stores - similar to the association between payment methods and stores currently in Spree master.

Default behaviour is to include shipping methods on the delivery step at checkout which have no stores selected, or the current store.

A store selector has been added to the admin --> shipping methods menu.

[mdavo6]

Hi @damianlegawiec , I believe you are currently in the process of merging this extension into Spree master. Would you like me submit a PR on the Spree repo with this functionality too?

[damianlegawiec]

Hi @mdavo6

It would be great to add it to Spree core, actually, we were planning to add this feature there for 4.3 :)