What about the established gateway providers such as www.coinpayments.com
POSaBIT company deals directly with cannabis dispensariea and has a decent UI in their PoS system. They use Litecoin only, as far as I know.
... and coins that have already built API's to be integrated into busineses' payment systems?
Haven't had time to look into all blockchains, but PRIZM crypto is the one with an open-sourse API - ready to be integrated in 2 clicks.
github.com/prizmspace/PrizmCore
A QR code is inside a wallet...
In regards to the target-audiences - all depends on the potential application and what the algorithm has to offer.
With Steem, as well as with PRIZM - where the barrier to entry is NOTHING - the potential audience is exaxtly that - EVERYONE, especially people in the emerging markets, stuck in poverty and hunger.
Coinpayments is also using gateways, they are not the gateway themselves.
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.