Classic ASP Requires Chilkat v11.0.0+
Shopify Set inventory levels to a certain amount

Use the set endpoint with the location ID and inventory item ID to set an inventory level to a specific value.
Chilkat Classic ASP Downloads

Classic ASP
<html>
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<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
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<%
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": 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",1)

url = "https://{shop}.myshopify.com/admin/api/2020-04/inventory_levels/set.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": 1,
'      "updated_at": "2018-06-26T15:44:33-04:00",
'  ...
'      "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>