digidiverdeb:is it legal for them to automatically put me on a credit account
Yes. If you don't pay, they can put put your details in a black folder and stick whatever colour stickers they like on the cover. There are a few abusive terms that they wouldn't be allowed to write on the drawer with similar folders in it but "credit accounts" is not one of them.
You haven't entered into a credit agreement though. All that has happened is that, by virtue of the money becoming owed, the company had become your creditor.
I agree that they should have communicated the situation a lot lot better. Any business that lasts will have found out that it is always best to try saying "please" a few times before getting nasty. A handful of repeat orders or recommendations to friends is always going to be worth more than the value of a single small purchase.