Why did the washing machine say $3.75 for a load & you could use your card (credit) if needed, which I did & it charged my acct $8.00. I just thought I would try it as I was from out of town and realized I didn't have the right bills on me for change

The credit card processing company will put a hold (charge) for $8.00 when you start a single machine. If you were to start a second machine for 3.75 then total spent would be $7.50 and you would be credit back 0.50 on your card. They do this to cut down on the transaction fees from MasterCard, Visa, etc. In the end, you will be charged the same whether your use cash or card. It is confusing but helps keep the costs down for convenience of using credit cards.

Upcharge for using plastic. It should be posted somewhere but it's not.

Not sure about that

I have no idea on the double charge .. Unless you selected extra wash extra rinse and hot extended and that's just s guess... Wish i could help more