By increasing days payable Amazon is able to fund their growth using suppliers' balance.
Key for Amazon is to have high inventory velocity, meaning they can collect from customers before payments to supplies are due. Inventory velocity is shown in Days Inventory Outstanding (DIO).
Days payable outstanding (DPO) is a standard accounting metric, showing company's average payable period. Days payable outstanding tells how long it takes a company to pay its invoices. The formula to calculate DPO is written as: ending accounts payable / (cost of sales/number of days).
This is caculated based on Accounts Payable and Cost of Sales figures reported by Amazon.
Last reported quarter 2018 Q3 it was 85.45, down by -1% year-over-year from 86.37.
Source: Amazon Quarterly Results.