From a customer point of view, I prefer to use credit cards, because of the charging flexibility and the consumer protection they provide. The latter is less of an issue when dealing with Action Electronics, but useful in other transactions!
Debit cards and cash require me to organise bank transfers or cash withdrawals, which mean I can't buy when I want to. Paypal is ok for ebay, and quicker than a credit card, but if anything goes wrong Paypal look after themselves before either the seller or the buyer, and are impossible to contact.....
Making an extra charge depending on payment type is a big no-no in my book, and I don't buy from places that do so. For a seller, the price advertised is the major basis of the agreement with the buyer, and re-negotiating that leaves a very bad taste.
There is a balance to be struck, of course. If I insisted on paying with a money order written on the side of a cow in Milton Keynes I can see that a seller would be put to some unreasonable expense to cash it. But the job of a buyer is to consider all the reasonable running costs of a business and reflect them in his prices.
Having said that, of course, there is equally no such thing as 'absorbing' the cost of a particularly difficult sale. Sellers do not have a cushion of extra money somewhere that they can use for this purpose. ALL the money they work with is provided by the buyers. When they say this, they are just noting that, on this sale, they will make less than average profit - maybe even a loss. And if they do this too often, they won't be in business. It's a bit like politicians offering a 'free' public service, paid out of taxes. Of course it isn't free - it just means that we are all paying...