Merchant Integration object deprecation

The Merchant Integration object will be deprecated and removed from the Return Details payload in 6 months.

This change is related to Two Boxes updating how we internally integrate to the 3rd party providers of merchants. We are decentralizing these integrations so that rather than integrations bundling merchant, site, return provider and product catalog provider, each can be a distinct integration.

Previously, each site that a merchant processed returns at required a distinct merchant integration to couple the merchant to the site. Going forward, merchants will be enabled at multiple sites directly. The result is that we no longer reply on merchant-integrations to determine the merchant and site of a return. Rather, Two Boxes will directly assign the merchant and site to the return itself.