I am developing an e-commerce mobiloe application using magento 2 rest apis only.This is the flow for making the REST API calls for order placement.
1.Create a cart 
api -->{{url}}/index.php/rest/V1/carts/mine 
This api will return a unique cart id
2.Add products to cart 
api --> {{url}}/index.php/rest/V1/carts/mine/items
body ->
{
 "cart_item": {
     "quote_id": cartId,
     "sku": skuName,
     "qty": 1
     }
}
3. Estimate Shipping Methods
url --> {{url}}/index.php/rest/V1/carts/mine/estimate-shipping-methods
body ->
{
    "address": {
        "region": "Trivandrum",
        "region_id": 12,
        "region_code": "CA",
        "country_id": "IN",
        "street": [
            "Amstor house",
            "Eramam"
        ],
        "telephone": "5656565454",
        "postcode": "670390",
        "city": "Kazhakuttam",
        "firstname": "Peter",
        "lastname": "K",
        "same_as_billing": 0,
        "save_in_address_book": 0
    }
}
This will return all possible shipping methods based on shipping address.In my case the result is
[
  {
    "carrier_code": "freeshipping",
    "method_code": "freeshipping",
    "carrier_title": "Free Shipping",
    "method_title": "Free",
    "amount": 0,
    "base_amount": 0,
    "available": true,
    "error_message": "",
    "price_excl_tax": 0,
    "price_incl_tax": 0
  }
]
4)Save shipping information
url --> {{url}}/index.php/rest/V1/carts/mine/shipping-information
body data ->
{
    "addressInformation": {
        "shipping_address": {
            "region": "Trivandrum",
            "region_id": 12,
            "region_code": "CA",
            "country_id": "IN",
            "street": [
                "Amstor house",
                "Eramam"
            ],
            "telephone": "5656565454",
            "postcode": "670390",
            "city": "Kazhakuttam",
            "firstname": "Peter",
            "lastname": "K",
        },
        "billing_address": {
            "region": "Trivandrum",
            "region_id": 12,
            "region_code": "CA",
            "country_id": "IN",
            "street": [
                "Amstor house",
                "Eramam"
            ],
            "telephone": "5656565454",
            "postcode": "670390",
            "city": "Kazhakuttam",
            "firstname": "Peter",
            "lastname": "K",
        },
        "shipping_method_code": "freeshipping",
        "shipping_carrier_code": "freeshipping"
    }
}
This will return all possible payment methods. Here i am using paypal_express for payment.
5. Payment using paypal plugin 
Here i will pay the amount using paypal cordova plugin.Also configured the IPN [{{url}}/paypal/ipn/]in paypal account
This api will return the following data,
{
   "client": {
       "environment": "sandbox",
       "paypal_sdk_version": "2.14.4",
       "platform": "Android",
       "product_name": "PayPal-Android-SDK"
   },
   "response": {
       "create_time": "2016-11-19T05:25:46Z",
       "id": "PAY-5VS11410F5341972MLAX6ETA",
       "intent": "sale",
       "state": "approved"
   },
   "response_type": "payment"
}
5.Save payment and place order
url --> {{url}}/index.php/rest/V1/carts/mine/payment-information
data ->
{
    "cartId": 3,
    "billingAddress": {
        "region": "Trivandrum",
        "region_id": 12,
        "region_code": "CA",
        "country_id": "IN",
        "street": [
            "Amstor house",
            "Eramam"
        ],
        "telephone": "5656565454",
        "postcode": "670390",
        "city": "Kazhakuttam",
        "firstname": "Peter",
        "lastname": "K"
    },
    "paymentMethod": {
        "method": "paypal_express"
    }
}
But this api will returning
{
  "message": "PayPal gateway has rejected request. Invalid token (#10410: Invalid token)."
}
Is there any api missing in the above flow for capturing payments.Please help me.