Thank you very much! Always a pleasure to be helpful! 

What error do you get?

Have you tried adding "()":

<assignvalue='source.{ORCgrp(1).RXE:7.1()}'property='target.{ORCgrp(1).RXE:7.1()}'action='set' />

When you create a RecordMapper to transform a CSV into an ObjectScript class you can add to the production a Business Process to receive the created object from the mapper, so you can transform it to XML (extending the ObjectScript class from %XML.Adaptor) and save it into a file created ad-hoc when you've received the first mapped row of the CSV (I guess that you can get from the original request the name of the original CSV file to identify the PONumber).

..SecondMethod

You can add after the %Save() command something like this:

set sqlUpdateMoodRecord = "UPDATE MoopRecord SET TotalAmount = 
        CASE WHEN TotalAmount > ? THEN TotalAmount - ? WHEN TotalAmount < ? THEN ? -TotalAmount
            ELSE TotalAmount WHERE %ID = ?"set statementUpdateMoodRecord = ##class(%SQL.Statement).%New()
set statusUpdateMoodRecord = statementUpdateMoodRecord.%Prepare(sqlUpdateMoodRecord)
if ($$$ISOK(statusUpdateMoodRecord)) {
    set resultSet = statementUpdateMoodRecord.%Execute(pRequest.TotalAmount, 
                        pRequest.TotalAmount, pRequest.TotalAmount, pRequest.TotalAmount,
                        tEntEpicMoopRecord.%ID)
}

Probably that code is not going to work...but the idea is to launch an UPDATE with a condition in the SET.

Have you assigned privileges to read the database of the namespace? 

Do you mean how to write the range in the OBX segment? I've this example of a range:

OBX|13|NM|MO^# Monocitos^SNM||0.00|#|0.2-0.9||||F|||20230330133551

Hi Scott! What application are you using to send the POST call? Is it a web application? IRIS?

Something like this example?
http://hl7.org/fhir/R4/allergyintolerance-fishallergy.json.html

{
  "resourceType": "AllergyIntolerance",
  "id": "fishallergy",
  "text": {
    "status": "additional",
    "div": "<div xmlns=\"http://www.w3.org/1999/xhtml\">\n      <p>allergy is to fresh fish. Tolerates canned fish</p>\n      <p>recordedDate:2015-08-06T00:00:00-06:00</p>\n      <p>substance:Fish - dietary (substance)</p>\n    </div>"
  },
  "identifier": [
    {
      "system": "http://acme.com/ids/patients/risks",
      "value": "49476535"
    }
  ],
  "clinicalStatus": {
    "coding": [
      {
        "system": "http://terminology.hl7.org/CodeSystem/allergyintolerance-clinical",
        "code": "active",
        "display": "Active"
      }
    ]
  },
  "verificationStatus": {
    "coding": [
      {
        "system": "http://terminology.hl7.org/CodeSystem/allergyintolerance-verification",
        "code": "confirmed",
        "display": "Confirmed"
      }
    ]
  },
  "category": [
    "food"
  ],
  "code": {
    "coding": [
      {
        "system": "http://snomed.info/sct",
        "code": "227037002",
        "display": "Fish - dietary (substance)"
      }
    ],
    "text": "Allergic to fresh fish. Tolerates canned fish"
  },
  "patient": {
    "reference": "Patient/example"
  },
  "recordedDate": "2015-08-06T15:37:31-06:00",
  "recorder": {
    "reference": "Practitioner/example"
  }
}