curl --request POST \
  --url https://sandbox.api.portal.io/public/proposals/{ProposalId}/items/{ProposalItemId}/sellprice \
  --header 'Accept: <accept>' \
  --header 'Content-Type: application/json' \
  --header 'X-MSS-API-APPID: <api-key>' \
  --header 'X-MSS-API-USERKEY: <api-key>' \
  --header 'X-MSS-CUSTOM-DATE: <api-key>' \
  --header 'X-MSS-SIGNATURE: <api-key>' \
  --data '
{
  "sellPrice": 583.78,
  "sellPercentage": 52.31,
  "sellPercentageBasis": "PartTotal",
  "setDefault": false,
  "updateAllInstances": false
}
'

import requests

url = "https://sandbox.api.portal.io/public/proposals/{ProposalId}/items/{ProposalItemId}/sellprice"

payload = {
    "sellPrice": 583.78,
    "sellPercentage": 52.31,
    "sellPercentageBasis": "PartTotal",
    "setDefault": False,
    "updateAllInstances": False
}
headers = {
    "Accept": "<accept>",
    "X-MSS-API-APPID": "<api-key>",
    "X-MSS-API-USERKEY": "<api-key>",
    "X-MSS-CUSTOM-DATE": "<api-key>",
    "X-MSS-SIGNATURE": "<api-key>",
    "Content-Type": "application/json"
}

response = requests.post(url, json=payload, headers=headers)

print(response.text)

const options = {
  method: 'POST',
  headers: {
    Accept: '<accept>',
    'X-MSS-API-APPID': '<api-key>',
    'X-MSS-API-USERKEY': '<api-key>',
    'X-MSS-CUSTOM-DATE': '<api-key>',
    'X-MSS-SIGNATURE': '<api-key>',
    'Content-Type': 'application/json'
  },
  body: JSON.stringify({
    sellPrice: 583.78,
    sellPercentage: 52.31,
    sellPercentageBasis: 'PartTotal',
    setDefault: false,
    updateAllInstances: false
  })
};

fetch('https://sandbox.api.portal.io/public/proposals/{ProposalId}/items/{ProposalItemId}/sellprice', options)
  .then(res => res.json())
  .then(res => console.log(res))
  .catch(err => console.error(err));

[
  {
    "id": 81376,
    "parentId": 42344,
    "itemType": "Part",
    "referencedItemId": 63140,
    "createdDate": "2030-08-21T00:22:19Z",
    "lastModifiedDate": "2031-02-06T00:22:19Z",
    "brand": "Sonos",
    "model": "Amp",
    "name": "Installation Labor",
    "shortDescription": "Sonos Amp",
    "clientNote": "Mounted in the rack behind the display.",
    "imageUrl": "https://images.portal.io/catalog/sonos-amp.jpg",
    "msrp": 901.23,
    "sellPrice": 583.78,
    "cost": 759.6,
    "costUpdateDate": "2030-11-10T00:22:19Z",
    "supplier": "Sonos Inc.",
    "quantity": 9.6,
    "total": {
      "amount": 982.49,
      "currency": {
        "code": "USD",
        "symbol": "$"
      },
      "isCombinedPrice": false
    },
    "isTaxExempt": false,
    "isRecurringService": false,
    "linkedOrders": [
      {
        "orderId": 21802,
        "orderNumber": 182,
        "orderNumberSuffix": "A",
        "supplier": "Sonos Inc.",
        "supplierRef": "SN-10420",
        "orderName": "Living Room Sonos Amp",
        "orderStatus": "Submitted"
      }
    ]
  }
]

{
  "responseStatus": {
    "errorCode": "BadRequest",
    "message": "Neither an absolute sell price nor a percentage was supplied, both were supplied, or a percentage was supplied without its basis."
  }
}

{
  "responseStatus": {
    "errorCode": "Unauthorized",
    "message": "Not Authorized. Ensure a valid session cookie or HMAC authentication headers are provided."
  }
}

{
  "responseStatus": {
    "errorCode": "PaymentRequired",
    "message": "The dealer's subscription is inactive or expired. An active subscription is required to use this endpoint."
  }
}

{
  "responseStatus": {
    "errorCode": "Forbidden",
    "message": "You do not have permission for this action."
  }
}

{
  "responseStatus": {
    "errorCode": "NotFound",
    "message": "Proposal or proposal item not found."
  }
}

{
  "responseStatus": {
    "errorCode": "Conflict",
    "message": "Proposal is in a state that does not allow editing."
  }
}