Hopefully this helps coming from a former credit analyst for a bank.
Opening the card is not necessarily a bad move. Sure, the interest rate is probably through the roof, but shouldnt kill your score.
Your best option is to leave the account open as the longer you have open accounts in good standing, the better it looks on your credit report. Closing one immediately does not look as good as having a reasonable limit and not using much of it.
You should try to charge one small item a month or every two months, and pay off the bill as soon as you receive it to avoid obscene service charges/interest. Try not to carry a balance, and doing so will not only show the bureaus you are responsible with credit, but it will show the merchant bank you do not need the card, and they are likely to increase your limit. All of these factors will actually increase your score.
The only negative aspect from simply opening the account is you will have an inquiry on your report, which provided you have not applied for credit repeatedly, does not particulaly fare badly for you.
Like I said, hope it helps.
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