Auto Sell BTC + Auto Buy & Send [Question]

We're about to launch a store that will sell digital and physical goods.

We're in need of a method to auto sell, buy and send BTC. Wanted to see if anyone could recommend the cheapest solution to this.

We've looked at Moonpay but the fees are pretty high to bank transfer to buy BTC.

https://support.moonpay.com/hc/en-gb/articles/360011930117-What-fees-do-you-charge-

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Exchange API is 'OK' but would love a more BITCOIN 'only' focused method that isn't going to have the SEC come shut down, etc.

Not looking for opinions on the state of things, just looking for an option that is the CLOSEST to BULLET PROOF.

Holding BTC isn't an option, it fluctuates too much for us to hold and send later, it could mean the collapse of our ecosystem. So we need the ability to make it stable via fiat or stablecoin, etc.

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Here is what we need to do:

1. Auto 'sell' BTC that is sent to any of our wallets : Can be sold for fiat, stablecoin, whichever to be held and withdrawn later
2. Auto 'buy' BTC and 'send' to user : This must be able to come from the 'stablecoin' or 'fiat' from 'selling' in step #1 or withdrawn from bank with 'low fees'. Lightning Network would be best for this I imagine, to only 'lightning wallets'.

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That's it!

Sounds simple but we've yet to find a solution, if you have please do share.

Thank you!