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I am talking about integrating Steem inside their system

They do have STEEM and SBD.

So what about all other gateways and services (e.g Shopify). There is an endless amount of them

Don't ask me, ask people who integrate this. In general, it's a very long process that can take up to 6-9 months simply because of regulation issues.

Shopify, for instance, has dozens of plugins that developers have already built over the years. Bitpay had it long time ago.

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