Woocommerce REST API not considering discounts and coupons

In your coupon_lines array, each item in the array needs 2 properties: code (the coupon code itself) and amount, which is either a flat fee (eg 50.00 off) or a discount (eg 10% off). Your structure ends up looking like:

"coupon_lines": [
    {
        "code": "MYCODE1",
        "amount": "10.00"
    },
    {
        "code": "MYCODE2",
        "amount": "25.00"
    }
]

If you’re already doing this – consider posting an example request here.