Several gun-related businesses were suddenly — and without warning — disrupted in recent weeks when Intuit stopped processing credit card payments because phone and internet sales were gun-related, The Post has learned.

Some of the payments stopped didn’t even involve firearms, but simply T-shirts and coffee mugs and gun safety classes, according to small business owners.

As a result, the businesses had to scramble to track down customers to get them pay their bills after Intuit credited back to customers’ accounts the purchases — even if the T-shirt was already shipped or the class already taken, one businessman told The Post.

At Gunsite Academy, a Paulden, Ariz., company that provides marksmanship training in addition to selling guns that ship to a licensed gun shop near the customer’s home, Ken Campbell was dinged by Intuit’s action.

Campbell, a former Indiana sheriff, had just switched credit card processors this spring — to Intuit, the parent of TurboTax and Quicken software — when the trouble began, he said.

Intuit told Campbell it mistakenly believed firearm sales were being made directly to the customers, which it requires for such items.

Campbell explained the guns were shipped to a local dealer with a federal firearms license who ran the required background checks. Intuit was unmoved.

Campbell said if he knew of Intuit’s stance, he would have simply moved back to his previous processor. “It’s fine, it’s capitalism, and if you don’t want to do business with us, we don’t want to do business with you,” he said.

Intuit did not return several requests for comment prior to deadline. It later stated, “As an American company, we respect the US Constitution and all the rights contained in it…For these transactions our bank partner requires them to be done face-to-face…Our policy today requires the customer to be present to swipe their credit card.”

This would mean, for instance, that it would process a firearm sale at a store, but not if the credit card was keyed in through the internet or over the phone.

Honor Defense, a Georgia firearms maker that ships only to other dealers, had a similar experience with Intuit.

Gary Ramey, president of Honor Defense, told Gun Talk radio host Tom Gresham that Intuit “reversed charges” on his customers as well.

Honor Defense could not immediately be reached for comment.