That javascript refresh is super handy, but it also hide error messages.
To debug,
Do this and then it will expose the success or failure message.
var text URL= URLRoot() & "db/" & Dbid() & "?a=API_AddRecord"
& "&_fid_15=" & [Related Sales order]
& "&_fid_13=" & URLEncode([Related Product])
& "&_fid_7=" & URLEncode([Product Description]) // this is a multi-line text box.
& "&_fid_8=" & [Product Quantity]
& "&_fid_9=" & [Product Unit Price]
& "&_fid_24=" & ToNumber([Taxable Item]);
$URL
//"javascript:" &
//"$.get('" &
//$URL &
//"',function(){" &
//"location.reload(true);" &
//"});"
//& "void(0);"
