You have misunderstood between Bitcoin as payment and Bitcoin exchange.
Well if you want to accept Bitcoin in your shop I think you need to consider the volatile price. Example situation: the customer buy 1kg of pet food at a price of $10 (0.00125 BTC after conversion, at the rate price of $8,000/BTC), and then you need to restock those pet food, let's say it's cost $8 and Bitcoin price fall down to $5,000. So your 0.00125 BTC become $6.25. You need extra funds to cover that, if you not restock those pet food, the circulation of your money will be hampered, isn't?


AFAIK since bitpay is no longer partnering with shapeshift, processing the payment using altcoin is currently not possible.
Doge coin is altcoin.