Hi! I have a stream-based JSON-to-XML converter of my own (however, is in Java, but I can port it if needed). My price/time is based on the assumption that you have (a relatively serviceable, simple-enough) data source or JSON resource/feed, and might be relatively straight-forward to transfer that over to its XML equivalent, so you put either a generated resource at your place/server or we provide a live feed there, so from the Merchant Center, you simply have it point to that URL for import? Currently, from your description, doesn't sound like you need to directly interface (or want some upload or some posting/submission via API) with the Merchant Center, so there might not be a need for authentication/credentials/login in your scenario?
For my bid, I'm also assuming you don't need too much of transformations, with that I mean not much "business logic", and primarily only a decent implementation for a format conversion, probably with some additional requirements in case it's a large amount/size of data, right?