Classic ASP
Shopify Update Inventory Level for a Product (Adjust available quantity)

Use the adjust endpoint with the location ID and inventory item ID to increase or decrease the available quantity for an inventory level:
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Classic ASP
<html>
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<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
</head>
<body>
<%
success = 0

' This example requires the Chilkat API to have been previously unlocked.
' See Global Unlock Sample for sample code.

set http = Server.CreateObject("Chilkat.Http")

http.Login = "SHOPIFY_PRIVATE_API_KEY"
http.Password = "SHOPIFY_PRIVATE_API_KEY"

' Also see: How to retrieve inventory levels

http.Accept = "application/json"

' The following JSON is sent in the request body:

' {
'   "location_id": 6884556842,
'   "inventory_item_id": 12250274365496,
'   "available_adjustment": 1
' }

' Use this online tool to generate the code from sample JSON: 
' Generate Code to Create JSON

set jsonRequestBody = Server.CreateObject("Chilkat.JsonObject")
success = jsonRequestBody.UpdateInt("location_id",6884556842)
success = jsonRequestBody.UpdateInt("inventory_item_id",12250274365496)
success = jsonRequestBody.UpdateInt("available_adjustment",1)

url = "https://{shop}.myshopify.com/admin/api/2020-04/inventory_levels/adjust.json"

set resp = Server.CreateObject("Chilkat.HttpResponse")
success = http.HttpJson("POST",url,jsonRequestBody,"application/json",resp)
If (success = 0) Then
    Response.Write "<pre>" & Server.HTMLEncode( http.LastErrorText) & "</pre>"
    Response.End
End If

Response.Write "<pre>" & Server.HTMLEncode( "Response Status Code: " & resp.StatusCode) & "</pre>"

set jsonResponse = Server.CreateObject("Chilkat.JsonObject")
success = jsonResponse.Load(resp.BodyStr)
jsonResponse.EmitCompact = 0
Response.Write "<pre>" & Server.HTMLEncode( jsonResponse.Emit()) & "</pre>"

If (resp.StatusCode >= 300) Then
    Response.Write "<pre>" & Server.HTMLEncode( "Failed.") & "</pre>"
    Response.End
End If

' Sample output...
' (See the parsing code below..)
' 
' Use the this online tool to generate parsing code from sample JSON: 
' Generate Parsing Code from JSON

' {
'   "inventory_level": {
'     "inventory_item_id": 12250274365496,
'     "location_id": 6884556842,
'     "available": 9,
'     "updated_at": "2018-06-26T15:04:49-04:00",
' ...
'     "admin_graphql_api_id": "gid://shopify/InventoryLevel/6485147690?inventory_item_id=12250274365496"
' ...
'   }
' }
' 

inventory_levelInventory_item_id = jsonResponse.IntOf("inventory_level.inventory_item_id")
inventory_levelLocation_id = jsonResponse.IntOf("inventory_level.location_id")
inventory_levelAvailable = jsonResponse.IntOf("inventory_level.available")
inventory_levelUpdated_at = jsonResponse.StringOf("inventory_level.updated_at")
inventory_levelAdmin_graphql_api_id = jsonResponse.StringOf("inventory_level.admin_graphql_api_id")

%>
</body>
</html>