which field in order object is changed when using Orders API PayOrder v2/orders/{order_id}/pay to pay zero total order?
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Additional Documentation
Take a Pickup Order and Pay for It
Payments API Integration
Orders API Overview
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but no tenders, and order status is open, so i do not know what is changed?
When PayOrder is called to pay for an order there are a lot of fields that change. What exactly are you looking at? 
I want to know if i can get receiptNumber and receiptURL, i mean it’s the case to pay $0 using PayOrder v2/orders/{order_id}/pay