If you have a loan from Amazon, payments to that loan are likely to show up in your settlements in A2X. A2X does not have access to the details of your loan so the split between principal and interest does not show up in your settlements. As this is the case, the amount of interest will need to be split out in your accounting system to record the expense.
See this Blog post for more information on Amazon Lending:
Amazon Lending Has Funded 20,000 Businesses