Debit cards were the cheapest to accept by far
That depends on the transaction amount.
When DP had a shop, well before the 'new breed' of card payments, it was a fixed fee for debit cards (about 40p) and a percentage of around 2% for credit cards with no minimum.
So if he took a small amount on a debit card, the fee could exceed his profit on a low price item - this is why some businesses set minimum limits. But the cost of a debit card for a larger purchase became more trivial.
For credit cards, it's fine for small items as it's pennies, but if it's a large purchase, the cost of accepting a credit card could be £20/30.
While not all businesses have a range of transaction amounts like this, it's important for the retailer to understand how they are charged for card payments and now it's possible to change to a service that's more cost effective.
If prices have gone up for all items, that could be because it's a place that has a lot of small transactions, and they have to allow for the type of people who'd buy a Mars bar or pint of milk with a card.
Personally, I pay with card most of the time, but I'm of an age where you were considered a bit of a twat for using a card for under about a fiver and most retailers wouldn't accept the payment anyway. Before that it was people paying by cheque for low purchases, usually because it was just before payday and they'd run out of money and knew that the cheque wouldn't be cashed until after they'd been paid.
So I do try to carry some cash and do use it for small purchases up to £2/3, plus there's also the times when you have to have cash, eg some parking meters or small retailers, plus the occasional time when the card system fails.