Splitit never charges any interest or fees.
However, interest, fees and/or charges may apply from your card issuer so your should always check this.
If you fall behind on your Splitit installment plan to the merchant or retailer, you could forfeit the right to pay in installments.
In these circumstances, the full outstanding amount may be charged to your chosen credit card using the latest authorization that was taken to guarantee your payments to the merchant or retailer. This could lead to charges from your credit card provider if you do not clear your balance in full - you should check this with them.