Most Bitcoin transactions should be contended on the Layer 2 mechanism, which will gave only seconds of transaction speed and a few fractions of a cent in terms of transaction costs. This is the best way in terms of a functional system of accepting Bitcoin.

Once the customer transacts a predetermined Bitcoin amount, the transaction can be logged and recorded by the retailer in a few seconds.

For other coins, there are similar approaches, but the transaction fees can be quite debilitating for a business.

Just a quick note before your proceed:

Choosing a cryptocurrency payment method for your customers is NOT something that should be obligatory. The only reason implementing such a system is to open a wider accepting audience willing to buy whatever merchandise. Do not force the customers hand in this situation, it'll just lead to messy results of 'whats Bitcoin' and yada yada.

As well, remember that cryptocurrencies are still a relatively niche community. Don't expect sudden retail growth as a result of such implementation of payment.

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DM me if you really want me to get you down into this sort of stuff. I personally do a lot of market analysis, been in the game for quite some time myself.