That's not what I would really consider a payday loan, which is only for a few weeks, usually. It really is a short-term installment loan, and hopefully it helped your scores a little bit.
I hope it helped too because I got two of them 3 months apart. I figured $15 for $500 was not bad since most places chareg you $30 for a $100.
How long a time was the loan? $15 on $500 for a year would be 3%, but of course if it's a shorter time the rate is higher. That's how a lot of these places get you sucked in (I would think a CU would be better). They tell you it's "only 10%." But if that 10% is for a week, it's 520% a year. If it's for two weeks, it's 260% a year. So you have to know not only the principle and interest, but also the term of the loan.