I don't think it's a kind of interest, but as a fee because this cash advance amount has to be paid from your future earnings. So, if you took $2000 as cash advance, your total payable amount would be around $2272. This amount would be deducted from your future earnings. If you could repay it in 1 month, that's great. But if it took one year or longer for you to repay, that's pretty fair as long as you can make more money using the cash advance. $272 for $2000 is about 13.6%.
From my experiences, I would recommend to take it if you can invest it somewhere else or if you can do another more profitable business with that amount. I have taken cash advance two times and I have used the money in another more profitable business, so for me, it helps a lot. I am waiting to get it again.
If you just want the money and have nothing profitable to do with it, I won't recommend taking it.
Thanks.