The Worst Mistake for Small Businesses to Continue in 2014 What is the one sure thing to turn away modern customers?

Now and then I come across that person who runs a business like it's still the 1960s. It's usually a restaurant, convenience store, gas station or a little shop. It doesn't seem to have been cleaned in years. There are dusty pictures of Sinatra and the Pope (Pope John XXIII, that is) on the wall. There's a cat or a sleeping dog somewhere. The floorboard creaks. There is little sunlight.

And yet, this person survives. He may in fact be doing OK. The food is still pretty good. There remains a loyal customer base and/or little competition in the neighborhood. The business is running on autopilot.

But he's making one big mistake:

"Sorry, we don't accept credit cards." Or, "We only accept credit cards for purchases over $10."

Do you ever hear this at Starbucks, 7-Eleven or Olive Garden? No. And these companies will ruthlessly take business from the 1960s guy if they can.

Here's a fact: many people aren't carrying cash anymore. You will have to accept credit cards, and soon you will have to accept electronic payments. Don't try to pretend that we, your customers, don't know the real reasons why you don't accept credit cards. You don't want to pay the extra fees, right? And you don't want the tax man to know how much you're bringing in, right?

Related: Survey: Small-Business Owners Still Slow to Embrace Credit-Card Payments

Don't you think the IRS is smarter than that? If they choose to audit you, don't you think a 12 year old could stand in your store or restaurant for a few days, tally up your average receipts and extrapolate over a year's worth of business to reasonably estimate your revenues? And if that number is significantly different than the one you're reporting because you're oh-so-smart and only taking cash and hiding receipts, don't you think you'll raise a few eyebrows?

Clinging to an all-cash business just to play games with your taxes is not a good model to follow. You will get burned.

Yes, there are fees. All the credit card companies charge them. And so do mobile payment services such as PayPal and Square. Some of them can be as high as 3 percent per transaction. And even though you can search around for a better deal, the fact is that you're going to pay fees.

So welcome to 2014. Remaining cash only may not only cost you new customers, but keeping the ones that you have. I may really like the veal parm at the little family joint in South Philly, but I'm getting tired of the "we only accept cash" sign that hangs in front of the register. There are plenty of other places in South Philly that serve a good veal parm who accept credit cards.

Cash is going away. We live in a world of credit cards, mobile payments, online banking, wire transfers and Bitcoin.

Sure, we miss Sinatra and Mickey Mantle. And my mom tells me a milkshake today just doesn't taste like it did when she was a kid. But even if you make that great milkshake, don't expect today's customer to have the cash to pay for it.

Related: What Your Banker Isn't Telling You